use dark colors for background and then use a light colors for the front of it.
That depends on who the birthday's for. You could use their top three favorite colors, or get festive with it and use all the colors in the rainbow.
Friendly colors
All colors.
She used all colors.
He showed his true colors when he did that.
all the colors
To remove colors, for paint, you use the paint remover. Or if that doesn't work, use color white-out. :)
Blue is a primary color, one of the colors you use to build others. You can not mix other colors to achieve blue (or red or yellow).
bright colours
Primary colors
Most artists only buy the primary colors and make all other colors from them