I have two different ways, to make whitewall tires.
The simple way to paint whitewalls is to take a Pigment gel Ink pen...
...and a circular template. Only choose the diameter, take a round:
Ready:
The same way works with some kind of wheels with those puits:
The second methode i use, like junkman said, is to paint the whitewalls with a paintbrush.
The only things i need is my paintbrush, paint, tape and a circular template:
Choose the diameter,...
...brush it...
ready.
Similar to this way, you also can paint a fine white wall:
The different way is:
First paint the rim black, lay down the circular template and choose a coin from your purse - on this above, it was a 2-Eurocent-Coin - to place in the center.