My first name “David” has five letters and begins and ends with the letter “D,” so I decided to fill my 25 boxes, horizontally and vertically, with letters from my first name. I used a font named Bodoni Black Squares. In the first image, I changed the foreground layer to black so all the letters would appear black, and I changed all the background to different colors — red, orange, yellow, green, and blue.
In the second image, I changed the color of letters, using the complementary colors of their backgrounds.
As suggested, here’s a new version without the black outlines: