Personally, I would take my fabric bundle to the local quilt shop. Find some colors that you think you would like and it it is gray choose several different shades - open the fabric (bolt) up and lay your april showers fabrics out on it. Then step back take a look, walk away from it and then turn around and look at it again. Depending on which Granny Squares pattern you are doing (I have one that has a med to dark brown as the background and it is a lot of little squares) when I looked online there are several different patterns with this name.
I would also look at a dark blue - it looks like there are only a couple of the dark blue prints in the line. Here is a link to not only the fabric but if you scroll down you can see some quilts made with the line and different background colors so that may help also. What ever you choose it will be wonderful, it is a really cute line.
http://www.unitednotions.com/fcc_april-showers.pdf