Because this wonderful quilt looks an awful lot like your inspiration.
(Actually, Veronika Mattison made it, according to Pinterest.)
No pattern -- but you really don't need one.
*Figure out your biggest square (piece it the same way as Log Cabin).
*Then do a two-square smaller set half the width and the same height as the larger block.
*A third block, the same width and roughly 2/3 height, can be added to the mix.
*Based on this quilt, Mattison also uses a thin sashing strip at times to separate and accent the color in that area.
Make a variety of blocks -- keep your colors carefully separate. (Using coordinated sashing helps, too.) Bear in mind that the quilt shown relies on the color wheel: primary colors are separated by the secondary colors that blend between them.
Then use this photo to lay the blocks out, rearranging and adding sashing strips as needed. Stitch together in sections until you reach the size needed for your quilt.
You won't need a fancy quilting pattern -- simple outline quilting will add depth and visual interest. Bind your finished piece with a dark neutral.
Your Rainbow Quilt will help keep things cheerful through the bleakest day of winter.