The only way to print white is to be a pro and have a pro printer with white ink...
For us (hobbyists), the only way to print white numbers is to use white decal paper, and print around the number to match the jersey color. In one word, in the graphics software, you make a rear layer of the jersey color, and pose white numbers in a front layer. Then, you print it on white decals paper so in fact you'll print around the number, making the illusion you printed in white.
The trick is to match the jersey painted color with your printed job...
What I would do : I'd scan a piece of paper with some jersey paint on it, then pickup the color in the graphics software. So the result should be pretty similar to the paint...but it's not an exact science if you don't have real pro tools.
It's why pro print graphic designers 's got 5inks print machines, and calibrated screens