I sold a baby quilt that was outline embroidery. Several weeks later the lady brought it back to show what she had done to it.
She said she had colored in the outlines with crayons. the used paper towels on top the pressed with a dry iron. the heat from the iron will melt the wax part of the crayon and leave the color in the fabric.
I haven't done this, but the colored quilt block really pop after being colored.