Yes you can Bleach a number on a hound and make it white.
You can also burn the hounds hair if you mix it to strong.
Use 30 Volume and it will work good.
The only problem with bleached numbers is if the hound runs through the creek or takes a dip in a mud hole, the number will become dull or non readable.
However, it is good to bleach a number on a hound then go over the bleached number with white paint, incase the paint gets messed up.
True Grit is right but as for myself I use 20 volume peroxide cause you can leave it on and it will not burn the hair and on black hair it may take three or four times to get the number real white.