BLACK black's symbol is made of clustered dots, to describe how black annihilates all other colors. Black does not appear in the rainbow, that is in the solar spectrum.
WHITE on the other hand, white's symbol is made of dots fanning out from a center, to show that white, like light, "expands", and evokes a feeling of open space. Like black, white doesn't appear in the solar spectrum, and is "made" by adding together all the colors in the rainbow.
RED the dots in red's symbol form a circle, meaning that red is a "full", "strong", "complete" color.
YELLOW the dots in yellow's symbol make a rhomb with nothing in its center; this is meant to show that yellow is an "open" color, but does not expand like white does, and maintains a distinct personality.
GREEN the dots in green's symbol describe a baseless triangle pointing upwards, a very simple image of a fir tree.
BLUE the dots in blue's symbol are disposed to form a wave, to represent the color of the sea reflecting a cloudless sky.
BROWN brown's symbol is made of dots describing a small hole in the ground, ready to welcome a plant's roots or a seed.
GRAY the dots in gray's symbol form a horizontal line, to represent both the color's "flat" and "quiet" character, and its "understated elegance".
PINK the dots in pink's symbol form a spiral to evoke, for instance, a rose bud or the heart of a large sea-shell.
PURPLE the dots in purple's symbol form a vertical line with two "legs" at the bottom, to represent the humility of the retiring violet.
ORANGE the dots in orange's symbol form a semicircle with its convex side facing upwards, to both show its affinity with red, and evoke the image of the setting sun in a clear day.
But beyond the meaning we attached to each symbol/sign, the primary concern remains that of building a code.