Depending on the shade of yellow, you might be able to use an offwhite or very light beige and have it blend well. Or a soft yellow might be nice, even with the Pooh blocks. What kind of quilting do you plan to do? If you're doing free motion and want to call attention to your stitches, you might even go with a slightly brighter yellow.
And there is always the option of using a transparent thread, if you don't want it showing up at all.
RST