You'll need paint in as many colors as you want, a foam brush and a wood board.
Sand the board if needed, including edges, brush water on the board to dampen it. This will help the paint blend better.
Squeeze out a line of each color and smooth it across the board, all the way to the edges. I cleaned out my brush after every couple of colors so that the colors wouldn't turn muddy, but I did like the look of blended colors where each one met another.
Let dry. (Bust out the hair dryer if you're impatient like me!)
Cut white vinyl (you can add whatever phrase you want of course) and place in the bottom right of the board. All done!
Style with a vase or some picture frames on your mantel. Enjoy your rainbow!
For more rainbow projects you might like
Rainbow Cake Pops and a Rainbow Stripe Quilt