diff --git a/README.txt b/README.txt
index 5ae7ddd..f333a3f 100644
--- a/README.txt
+++ b/README.txt
@@ -28,6 +28,9 @@ lorenz - a Lorenz attractor
 primes - display primes as rod numerals (hommage to Rune Mields)
     parameter is the number of pixels scrolled per second
 
+colorcode - Show a color coded resistor
+    parameter is not used yet
+
 Compile with "cargo build --release".
 
 The binary executables are in the "target/release" subdirectory.
diff --git a/src/bin/colorcode/main.rs b/src/bin/colorcode/main.rs
new file mode 100644
index 0000000..e8d745a
--- /dev/null
+++ b/src/bin/colorcode/main.rs
@@ -0,0 +1,292 @@
+use std::collections::HashMap;
+use std::env::args;
+use std::io::stdout;
+use std::io::Write;
+use std::thread::sleep;
+use std::time::Duration;
+
+use rand::thread_rng;
+use rand::Rng;
+
+use lowdim::bb2d;
+use lowdim::p2d;
+use lowdim::Array2d;
+use lowdim::BBox2d;
+use lowdim::Point2d;
+
+use pixelfoo_apps::color::Color;
+
+type Error = Box<dyn std::error::Error>;
+type Result<T> = std::result::Result<T, Error>;
+
+const COLORS: usize = 3;
+
+#[derive(Clone, Debug)]
+struct Frame {
+    pixels: Array2d<i64, Color>,
+}
+impl Frame {
+    pub fn new(bbox: BBox2d, background: Color) -> Frame {
+        let pixels = Array2d::with(bbox, |_| background);
+        Frame { pixels }
+    }
+    pub fn bbox(&self) -> BBox2d {
+        self.pixels.bbox()
+    }
+    pub fn set(&mut self, pos: Point2d, pixel: Color) {
+        if self.bbox().contains(&pos) {
+            self.pixels[pos] = pixel;
+        }
+    }
+    fn set_rectangle(&mut self, bbox: BBox2d, color: Color) {
+        for p in bbox {
+            self.set(p, color);
+        }
+    }
+}
+
+fn send<T: Write>(w: &mut T, frame: &Frame) -> Result<()> {
+    for y in frame.bbox().y_range().rev() {
+        for x in frame.bbox().x_range() {
+            let color = frame.pixels[p2d(x, y)];
+            w.write_all(&color.rgb())?;
+        }
+    }
+    Ok(w.flush()?)
+}
+
+const DEFAULT_ARG: usize = 0;
+
+#[derive(Clone, Copy, Debug)]
+enum Ring {
+    Black,
+    Brown,
+    Red,
+    Orange,
+    Yellow,
+    Green,
+    Blue,
+    Violet,
+    Gray,
+    White,
+    Silver,
+    Gold,
+}
+impl Ring {
+    #[allow(unused)]
+    fn value(&self) -> i64 {
+        match self {
+            Ring::Black => 0,
+            Ring::Brown => 1,
+            Ring::Red => 2,
+            Ring::Orange => 3,
+            Ring::Yellow => 4,
+            Ring::Green => 5,
+            Ring::Blue => 6,
+            Ring::Violet => 7,
+            Ring::Gray => 8,
+            Ring::White => 9,
+            Ring::Silver => -2,
+            Ring::Gold => -1,
+        }
+    }
+    fn color(&self) -> Color {
+        match self {
+            Ring::Black => Color::new(0, 0, 0),
+            Ring::Brown => Color::new(165, 82, 42),
+            Ring::Red => Color::new(255, 0, 0),
+            Ring::Orange => Color::new(255, 165, 0),
+            Ring::Yellow => Color::new(255, 255, 0),
+            Ring::Green => Color::new(0, 255, 0),
+            Ring::Blue => Color::new(0, 0, 255),
+            Ring::Violet => Color::new(238, 130, 238),
+            Ring::Gray => Color::new(190, 190, 190),
+            Ring::White => Color::new(255, 255, 255),
+            Ring::Silver => Color::new(220, 220, 230),
+            Ring::Gold => Color::new(255, 215, 0),
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+struct Resistor {
+    rings: Vec<Ring>,
+}
+impl Resistor {
+    fn new(rings: Vec<Ring>) -> Resistor {
+        Resistor { rings }
+    }
+    fn rings(&self) -> &[Ring] {
+        &self.rings[..]
+    }
+}
+
+const E12: &[[Ring; 2]; 12] = &[
+    [Ring::Brown, Ring::Black],
+    [Ring::Brown, Ring::Red],
+    [Ring::Brown, Ring::Green],
+    [Ring::Brown, Ring::Gray],
+    [Ring::Red, Ring::Red],
+    [Ring::Red, Ring::Violet],
+    [Ring::Orange, Ring::Orange],
+    [Ring::Orange, Ring::White],
+    [Ring::Yellow, Ring::Violet],
+    [Ring::Green, Ring::Blue],
+    [Ring::Blue, Ring::Gray],
+    [Ring::Gray, Ring::Red],
+];
+
+fn random_element<'a, T, R: Rng>(elements: &'a [T], rng: &mut R) -> &'a T {
+    let i = rng.gen_range(0..elements.len());
+    &elements[i]
+}
+
+fn main() -> Result<()> {
+    let args = args().collect::<Vec<_>>();
+    eprintln!("executing {}", args[0]);
+
+    let x_size = args[1].parse::<usize>().unwrap();
+    let y_size = args[2].parse::<usize>().unwrap();
+    let arg = if let Some(s) = args.get(3) {
+        s.parse::<usize>().unwrap_or(DEFAULT_ARG)
+    } else {
+        DEFAULT_ARG
+    };
+    eprintln!("screen size {}x{}, arg {}", x_size, y_size, arg);
+
+    let mut rng = thread_rng();
+
+    let frame_size = x_size * y_size * COLORS;
+
+    let x_size = i64::try_from(x_size)?;
+    let y_size = i64::try_from(y_size)?;
+
+    let bbox = bb2d(0..x_size as i64, 0..y_size as i64);
+    let mut frame = Frame::new(bbox, Color::black());
+
+    let metal_body_color = Color::new(0, 192, 192);
+    let carbon_body_color = Color::new(242, 200, 150);
+    let wire_color = Color::new(180, 180, 190);
+
+    let delay = Duration::from_millis(1000);
+
+    let y_mid = (bbox.y_min() + bbox.y_max()) / 2;
+
+    let y_wire_min = y_mid;
+    let y_wire_max = y_mid + 1;
+    let wire = bb2d(bbox.x_range(), y_wire_min..=y_wire_max);
+
+    let x_body_size = 56;
+    let y_body_size = 12;
+    let x_ring_offset = 8;
+    let x_ring_extra_offset = 4;
+    let x_ring_size = 4;
+    let x_ring_space = 4;
+    let x_ring_extra_space = 4;
+
+    let x_body_min = (x_size - x_body_size) / 2;
+    let y_body_min = (y_size - y_body_size) / 2;
+    let x_body_end = x_body_min + x_body_size;
+    let y_body_end = y_body_min + y_body_size;
+    let body = bb2d(x_body_min..x_body_end, y_body_min..y_body_end);
+    let mut body_color = metal_body_color;
+
+    let mut rings_variants = HashMap::new();
+    for n in 4..=5 {
+        let mut rings = Vec::new();
+        for i in 0..n {
+            let x_ring_min = x_body_min
+                + x_ring_offset
+                + if n == 4 { x_ring_extra_offset } else { 0 }
+                + i * (x_ring_size + x_ring_space)
+                + if i == n - 1 { x_ring_extra_space } else { 0 };
+            let x_ring_end = x_ring_min + x_ring_size;
+            let ring = bb2d(x_ring_min..x_ring_end, y_body_min..y_body_end);
+
+            rings.push(ring);
+        }
+        rings_variants.insert(usize::try_from(n)?, rings);
+    }
+
+    let p_pruefungsfragen = 0.1;
+    let pruefungsfragen = vec![
+        Resistor::new(vec![Ring::Brown, Ring::Red, Ring::Red, Ring::Gold]), // NC103
+        Resistor::new(vec![Ring::Red, Ring::Violet, Ring::Red, Ring::Gold]), // NC104
+        Resistor::new(vec![Ring::Yellow, Ring::Violet, Ring::Red, Ring::Silver]), // NC105
+        Resistor::new(vec![Ring::Red, Ring::Violet, Ring::Orange, Ring::Gold]), // NC106
+        Resistor::new(vec![Ring::Yellow, Ring::Violet, Ring::Orange, Ring::Silver]), // NC107
+        Resistor::new(vec![Ring::Green, Ring::Blue, Ring::Red, Ring::Silver]), // EC112, EC113
+    ];
+
+    let t_solve = 10;
+    let t_end = 15;
+
+    let mut t = 0;
+    let mut r = Resistor::new(vec![Ring::Brown, Ring::Black, Ring::Red, Ring::Gold]);
+    loop {
+        if t == 0 {
+            // Pick a new resistor.
+            if rng.gen::<f64>() < p_pruefungsfragen {
+                r = random_element(&pruefungsfragen, &mut rng).clone();
+                body_color = carbon_body_color;
+            } else {
+                let is_five_ring = rng.gen::<f64>() < 0.5;
+
+                body_color = if is_five_ring {
+                    // A turquise blue metal film resistor.
+                    metal_body_color
+                } else {
+                    // A brownish beige carbon resistor.
+                    carbon_body_color
+                };
+
+                let mut rings = random_element(E12, &mut rng).to_vec();
+                if is_five_ring {
+                    // Add a black ring for a five-ring resistor.
+                    rings.push(Ring::Black);
+                }
+
+                let r3s = [
+                    Ring::Black,
+                    Ring::Brown,
+                    Ring::Red,
+                    Ring::Orange,
+                    Ring::Yellow,
+                ];
+                rings.push(*random_element(&r3s, &mut rng));
+
+                let r4s = if is_five_ring {
+                    [Ring::Brown, Ring::Red]
+                } else {
+                    [Ring::Gold, Ring::Silver]
+                };
+                rings.push(*random_element(&r4s, &mut rng));
+
+                r = Resistor::new(rings);
+            }
+        }
+
+        // Display the resistor.
+        frame.set_rectangle(wire, wire_color);
+        frame.set_rectangle(body, body_color);
+        let ring_bboxes = &rings_variants[&r.rings().len()];
+        for (&ring_bbox, ring) in ring_bboxes.iter().zip(r.rings().iter()) {
+            frame.set_rectangle(ring_bbox, ring.color());
+        }
+
+        if t >= t_solve {
+            // TODO Display the resistor value as text.
+        }
+
+        let mut buf = Vec::with_capacity(frame_size);
+        send(&mut buf, &frame)?;
+        stdout().write_all(&buf)?;
+        stdout().flush()?;
+        sleep(delay);
+
+        t += 1;
+        if t >= t_end {
+            t = 0;
+        }
+    }
+}