1. When you're out of colors that you want mix them
2, when you run out of things to mix them with, switch media.
I start working by doing paintings with tube acrylic paints, when I run out of those I go back to using bottle acrylics on wood with either furniture projects, bird houses, boxes or dala horses.
When I run out of those there's always watercolors or colored pencils on a variety of different papers.
If I run out of those too it's on to found objects or junk mail. the main thing is that if you're motivated enough there's always a way to do something creative.
Don't wait for that perfect moment or think that you have to have something special to work with, just live in the moment and grab whatever you've got to make your world a better place.