How about rail fence? It would be pretty easy to sew using 2.5" strips together to give you 6" blocks of 3 colors, and there are many ways to vary the layout. If you have a lot of one color (for example, white), you can sew just the middle strip in colors. Or, 6 strips could be sewn together to form 12" blocks. You can also add sashing between the blocks if desired.
Here are some examples of rail fence possibilities on Pinterest:
https://www.pinterest.com/search/pin...yped&term_meta[]=rail%7Ctyped&term_meta[]=fence%7Ctyped&term_meta[]=quilt%7Ctyped
My other thought is the Falling Charms quilt:
https://www.youtube.com/watch?v=4KkpbFKBaKA
The only thing about Falling Charms is that you really need a single background fabric for each quilt. With rail fence, you just need to assemble 3 to 6 fabrics (or a collection of scrap fabrics) that work together.