I am going to geek out and allow the OCD, math, and logic in me run free:
1, 2, 3 are primary colors
1 = Yellow
2= Blue
3 = Red
Using a purely math progression:
4, SHOULD BE: 1 + 2, Yellow + Blue = Green;
5 SHOULD BE : 1 + 3, Yellow + Red = Orange;
6 SHOULD BE 2 + 3, Blue + Red = Purple
7...