I guess I'm pointing out the obvious, but couldn't you just spray paint the joystick ball to whatever color you please?
Sure you could, as long as you never planned on actually using the stick. Otherwise the paint would wear off, and quickly.
Of course I totally don't understand the obsession with "I need my stick this color", since nearly every game ever made had either red or black sticks. Of course there are some exceptions, but if you ever look in someones parts box (check mine if you wish), you will find almost nothing but red and black sticks.
Now, with all that said. I wouldn't mind a pair of white Wico balltops, but I am not going to go special ordering.