DOUBLE or OVERLAYED STICKERS

The first step for deciding colors is to determine the background color of where you are putting the vinyl.

What color is your wall or the board you are sticking it to?

For a light colored wall or board, choose a lighter color for your background words.
For a dark wall or board, choose a dark color for the bottom words.

Then choose a contrasting color for the top layer. 
Like in the samples above.

Many like to layer red letters on black letters.  From a design standpoint.... it's not a good idea. Just not enough contrast.
But hey.... you can do whatever you want to... after all... it's all about YOU!