Add random_color and the color palette

random_color returns an immutable (red, green, blue) tuple of integers from 0 to 255
and random_palette_color picks a named color. Both take an optional random.Random
so that tests can seed them.

Task: MIL-003#1
Task: MIL-003#2

src/turtle_challenges/colors.py

@@ -0,0 +1,44 @@
+"""!
+@file colors.py
+@brief Random colors for the challenges.
+
+A color is either a name from the palette (challenge 3) or an RGB tuple
+(challenges 4 and 5). A tuple is the right type for an RGB value: it is
+immutable, so a color cannot change after it is made.
+"""
+
+import random
+
+from turtle_challenges.constants import (
+    COLOR_CHANNEL_MAX,
+    COLOR_CHANNEL_MIN,
+    COLOR_PALETTE,
+)
+
+## Generator used when a caller passes none; tests pass a seeded one instead.
+_DEFAULT_RANDOM = random.Random()
+
+
+def random_color(rng: random.Random | None = None) -> tuple[int, int, int]:
+    """!
+    @brief Pick a random RGB color.
+
+    @param rng Random generator to draw from; a shared one is used if omitted.
+    @return A tuple of red, green and blue, each from 0 to 255.
+    """
+    generator = rng if rng is not None else _DEFAULT_RANDOM
+    red = generator.randint(COLOR_CHANNEL_MIN, COLOR_CHANNEL_MAX)
+    green = generator.randint(COLOR_CHANNEL_MIN, COLOR_CHANNEL_MAX)
+    blue = generator.randint(COLOR_CHANNEL_MIN, COLOR_CHANNEL_MAX)
+    return (red, green, blue)
+
+
+def random_palette_color(rng: random.Random | None = None) -> str:
+    """!
+    @brief Pick a random color name from the palette.
+
+    @param rng Random generator to draw from; a shared one is used if omitted.
+    @return One of the names in `COLOR_PALETTE`.
+    """
+    generator = rng if rng is not None else _DEFAULT_RANDOM
+    return generator.choice(COLOR_PALETTE)