Don't try to DO anything with the colors but use them!
Browns or black would really mess the happy quality of the blocks up. Find the same colors in polka dots, or stripes, or florals and go with the flow! If you really don't like it for yourself, think how happy a child would be to receive such a happy quilt! I have made baby quilts using all different brights and always binding them in black and white zebra print and the babies eyes can follow the colors. Enjoy the joy of the colors and make someone else happy with it. You have done a lovely job on the blocks!