Of course, they are unpainted. How can I paint/ink/color the numbers to get very high contrast between the numbers and the die?
I experimented with the methods proposed (except liquid white-out; couldn't find any), with the following results:
Paint: I used acrylic paint to fill in the numbers and paper towel to remove the excess. Very good opacity with this method, but I could not get enough paint to stay in the numbers during the removal of the excess. I tried various drying times before removing the excess, but could not get the results I wanted. Maybe another type of paint would work better?
Marker: I used Sharpie ultra-fine point white marker. My hands are not as steady as they used to be, so I had a hard time getting a neat and tidy finished appearance. The markings were not very opaque and did not provide great contrast. (I'm guessing that black marker on light colored dice would provide much better results.)
Crayon: I experimented with several brands and types of crayon. Crayola seems to work the best, probably because of the higher quality wax, which is softer and clings in the grooves of the numbers better. Specifically the Crayola GelFX crayons worked the best. They are made for writing on black paper, so the pigments are very opaque, and the wax is very soft and sticky; really stays in the numbers. Time will tell if it holds up to heavy use.
Below is an image of two of the dice filled with Crayola GelFX 'Spring Green' and 'Tickle Me Pink'. (The girls thought white was booooring).
It's coming up on 2 years now (21 months) since I used the crayon on these dice. They have had moderate to heavy use in that time. The color has shown no signs of wear or fading, and it is staying in the numbers right where I put it.
Conclusion: Crayola Gel FX crayons are the way to go. Color goes on easy, fills well, and is very opaque.
The traditional technique is to rub a crayon repeatedly across the number and then wipe away the considerable excess with a tissue or paper towel. That's why old boxed sets use to come with crayons.
(but that way is silly)
First, wash the dice. Then use a fine brush to ink the numbers with white paint, then quickly use a paper towel to wipe off the excess.
I used miniatures paint most recently on some new black Gamescience dice. As others attest, paper towel just smears paint everywhere and sucks the paint out of the numbers. Turns out that there is a simple solution: use your fingers. When I switched to using my fingers, I quickly got clean faces and paint filled numbers. On the down side, my fingers were painty, but with water soluble paint it cleaned up easily.
You could use the stuff that is made for the job:
One brand, called Lacquer-Stik, is available from Amazon or Brownells. Check the prices and color availability before you order. Or, check around a little on your own. Those were just the first two places I found when I searched.
It comes in black, gold and bright colors. It's inexpensive and permanent.
I had bad luck painting the Gamescience dice with miniature paints. My errors stuck to the dice and smudged when I tried to remove them. I didn't have this problem fixing up chessex dice.
Depending on the color of the dice, mechanical pencil is not unreasonable. It's pretty shiny and it's hard to screw up. Not as bright as actual paint though.