The colors are very nice. I might suggest a varigated thread with a meander quilt pattern.

What is a meander quilting pattern?

I would use variegated for sure ... Superior Threads makes one that would be great -- King Tut Joseph's Coat

Smoky invisible thread. I've used it a lot and have had no problem with it. Works well for stitch in the ditch or diagonal machine quilting. I don't know about free motion or meandering with it though. It holds up well in laundering. The bobbin thread should match the back of the quilt but the invisible thread will not be seen on the top. Use the smoky one for darker colors and the clear one for lights.

Since you have used so many colors of fabric, just about any of them would work well for quilting, however, matching the backing is my preferred color choice. There is nothing wrong with stitching in the ditch. Holds a quilt just wonderfully. Cross hatching would be lovely also - diagonal lines in both directions.

By the way...if you had fun making the quilt and put your love into making it...it is worthy. If someone gets to wrap up in it and feel the love or enjoy the warmth it provides...it is worthy.

I'd an all over design in a multi colored bright variagated thread. You're squares will have another whole dimension!

I would use a varigated or a gray because grays tend to blend, I would quilt it with crosshatch, look up a good tutorial on basting and quilting and go for it.

Gold would be bold. LOL really gold would look great. and all over design to quilt it. something with curves and flows. I do love the quilt. right up my ally

Quilt 1/4" inside each block.
That will accentuate the blocks.
J J

You could combine the decorative stitches and meandering. Since you have never meandered, you might feel comfortable doing a decorative around maybe 4 blocks and then meander in between them. It would be easier then doing an all over meander.
If you GDD likes purple, go for it. I personally would like a varigated thread.