So you've got your dice and (for whatever reason) want them in a different color than the slightly off-white they're in. Or, you want the numerals to be much more readable. Good news! You have a few options for both problems.
NOTE: Putting your dice in an environment above 90°F is NOT recommended, and will melt the dice, destroying the braille.
Method 1: For the entire die, as well as a good way to do the entire set at once, try Rit brand dye, and follow the steps shown here.
Method 2: Grab a sharpie or other permanent marker and color the sides of the die as you wish. Be careful, as the ink may stain anything it touches, and needs some time to dry. This method is great if you want every side to be a different color.
Method 3: With a crayon, rub the wax onto the face of the die. Note that this WILL fill up the indented numeral, so try to choose a color that will let it still be visible (unless you don't want that). Also note that this will NOT fully color the die, the wax will fill up small cracks, but usually leaves a blotchy stripe pattern. It is however, great with the sharpie method. Color in the numeral with one color, then use a sharpie to color the rest using the method above.
Method 4: To fill in the numerals a bit: use a super-fine pen (recommended) or a mechanical pencil (may smear) to color in the bottom of the numerals. Be very careful, as the lines of the numerals may be low, and it is very easy to get marks on the rest of the die face.
Look for the dice to be up as soon as the backer rewards are fulfilled!