I want to colour the US (only the states) map with Yellow, Green, Red and Blue. I was wondering what would be the lowest number of states with the colour of Green. We can of course use the other colours as much as we want. Please note that I want to follow the Four Color Theorem rules.
Motivation: I am studying graph theory and I want to know if there is a way that we could limit the use of the fourth colour as much as possible. This is not a homework problem.
My attempt: I have tried many variations and can limit it to 6 and it seems like the minimum possible but there are many possibilities to try ($4^{50}$). Therefore I was wondering if there is a simpler method? Thank you in advance.
Clarification: I am interested in only the mainland of USA. For states like Michigan that are split, I used the same colour for both parts.