@Jen Aside,
B is the second letter. So B=2 E is the fifth letter, so it equals 5. 5-2=3, which is the number of letters in RED. (B and E are the letters for red, obviously)
M is the 13th letter. R is the 18th letter. 18-13=5 which is the number of letters in green.
This pattern doesn't continue for orange, however. Unless you add. A is the first letter. A=1. E is the 5th letter. E=5. 1+5=6=number of letters in orange.
Don't know if this helps.. But it's all i've got so far.
Also, orange and blue are both even numbered in the number of letters in their name, so i'm thinking maybe they both add? And odd numbered color names subtract? 3+5=8=number of letters in "subtract".