...when there are 52 shades of each?

I've been borrowing other people's ideas for plant combinations with good results but now I have enough plants and shrubs already in the ground that finding a plant to add nearby--that won't clash--is a real challenge.

How do you figure out *which* color for the new addition will work when all you have to go on is a catalog photo? Or, you're at the greenhouse and need to figure out which "purple" is the *right* purple to go with the plants back at home???

The perfect example of my latest misadventure is going on right now in my yard: the alliums that were *supposed to* complement the heuchera and barberry are the wrong color of purple! HELP!