well, I am confused right away by your objective function...

Why are you maximizing 120B+120Y+120G ?

Since you are selling the paint at $100 per ton, shouldn't it be max 100B+100Y+100G ?

and I'm not sure, but your constraints might need to look like this:

(note: The inequalities here are not strict...so when I use <, I actually mean less than or equal to)

2B+Y < 6

2B+2Y+G > 8

2B+2Y+G < 10

B,Y,G > 0

If this looks like it makes sense, I can help you solve it...If I have misunderstood, I apologize...and please clarify