solid color balls are 1-7, they're all smaller numbers than the 8 ball which is the middle ball of the 15. 9-15 are striped.
IIRC (I haven't darkened the door of a pool hall lately)
1 yellow
2 blue
3 red
4 purple
5 orange
6 green
7 dark red (carmine?)
Originally, all balls were solid. Lows were red, highs were yellow, middle (the 8-ball) was black. When the game went away from the British style, lows became solids, highs became stripes, and the 8-ball remained the solid black.
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