How do I test foundation colour at the store?

You want to put a somewhat thicker than normal swatch in a vertical line on your jaw bone, just before it becomes your neck.  This colour should sit there and dry, if necessary.  It should match your neck colour as best it can and almost disappear.  Then you know you have the right colour.  Go outside in the natural light if you need to before you buy.  Most lines will let you return it if you are not satisfied with the colour.  You can also take a tester home to play around with.