what do you want to POP? the color you add or the pastels that are already there. To me, making something POP means that you are bringing out the value/presence of what is already there so, with the right medium to dark, the pastels will really pop, which should please the recipient in this case. I'd just be careful in selecting the med/dk fabric carefully...and a little goes a long way. How about a narrow flange around blocks or around the quilt - only about 1/8 inch visible? this would highlight the pastels by giving them a very small amount of contrast to be compared to.