Invisible thread is more difficult to work with than 100% cotton thread. Learned that the hard way, of course. If you pressed your seams to one side, a 50wt 100% cotton (I love Aurifil brand, but if you're looking to save money, 100% cotton Gutermann will work okay; Aurifil blends in more with the fabric because it has 2 strands instead of the standard 3) thread in black would hide your quilting the best with SITD.
That sashing -- with a black thread -- would also be a great place to practice free motion quilting. The only way to learn new skills is to start doing them. I think stained glass quilts are the ideal quilt for trying out different types of free motion stitches, but that's up to you. I think a pale yellow could look really nice, or you could use an invisible thread for that. It won't show up that much, so you could make plenty of mistakes & most people would never notice. The downside with the invisible thread I've found is with unsewing; I need to go one size up on my needles & on top of that I struggle a bit more with picking it out. But maybe that was just the brand I chose. I don't know. Just make sure not to put invisible thread in the bobbin (learned that the hard way, too).