Use Chalk: Check out the letters in the title of my "First Bike" layout. All but one of them are from the same sticker sheet (BasicGrey 'Cupcake'). Can you tell which letter is from a different set?
It's the T. It's from BasicGrey 'Oh Baby Boy.' It was a similar color, but definitely not close enough. You can see it here:
Use Paint: Check out the chipboard letters in my "Paint" layout. They were all different colors and patterns that clashed with one another, my photos and my paper. I painted the letters with a cream base coat, then used the back of the paintbrush to add dots of all different colors. Now they not only go together, but they reinforce the theme of the layout.