Paintballs are one of the parts of hexing that has been said to be able to drive anyone mad! I'm not exactly an expert on them myself, but I can give the basic gist.
This tutorial is written for LNZ Pro. It will also work very similar in ResHack, but not so well in older hex programs, though you may still get something out of understanding how it all works, perhaps!

The paintball section looks something like this:
[Paint Ballz]
;base ball diameter(% of baseball) direction color outlinecolor fuzz outline group texture
24, 45 1, -.9, -.9 25 15 2 -1 2 1 0
24, 45 -.9, -.9, -.9 45 15 2 -1 2 1 0
16, 50 0, 1, .3 37 -1 0 -1 -1 -1
17, 50 0, 1, .3 37 -1 0 -1 -1 -1

At the top, there is a little list, see it? This list tells us what each number means - it's very helpful! I will now go through what each one means more closely.

1 - Base ball. The base ball is the ball you want to place the spot on. So if you want a ball on the head, you put in 52 for dogz and 24 for catz, and so on.
2 - Size. This is how big the ball will be. The number you put in is not an absolute, it's a percent. So if you put in say 50, the spot will be 50 % the size of the base ball.
3 - This number says how far to the left or right the spot will go. For the numbers, the extremes go between -9 and 9. Higher/lower numbers than these won't make a difference. In this column, 1 is furthest to the left, -9 is farthest to the right. All other numbers are between them. 0 of course is in the middle.
4 - This tells the program how far up or down the spot will be. 9 is at the bottom, and -9 at the bottom, with the others inbetween. 0 in the middle.
5 - And this finally is how far forward/back the spot will be. 0 is middle, -9 is front, 9 is back and the others inbetween. To get a spot exactly where you want it, you'll probably need to work with all three "position numbers". I usually begin to put all at 0, then gradually work through each one until the spot is as I want it.
6 - Colour. Pretty self-explaining!
7 - Outline colour. Not too hard to understand either! Normally you want the spots to have the same outline colour as the spots themselves (ie appearing to have no outline). Then just put -1, that tells the game to pick up the colour of the spot itself.
8 - Fuzz. How much fuzz the ball will have. I recommend to keep it at least 1 for most spots. 0 looks perfectly round, not so natural! Above 3-4 tends to look scruffy rather than fuzzy.
9 - Outline. Most of the time, you keep this at -1 which means "no outline". 0 and -2 are both half outlines (appear on different sides). 1 is a full outline, and then higher numbers are thicker version of 1. So-called leopard spots are spots that have thicker outlines.
9 - Group. This has to do with 2nd gen colour inheritance. You can ignore this most of the time. It's mostly for experied breed hexers to deal with.
10 - This one can confuse people sometimes! This is where you change texture. Basically you put in the texture # here to get the one you want. If you want more info on this, go to my texture tutorial (opens in a new window so you won't have to press Back to get back here). Look under Assigning textures.

Here are some common paintball-related problems.
- If the spot isn't showing up, make sure that A) that part of the base ball isn't covered up by another ball - if it is on the head, perhaps the ball is placed right under where an ear ball is. B) that it is not below another spot. In the paintballz section, if I recall it right, the spots in the top of the section will be placed on top of the lower ones. So if you for instance have tan markings and a chest patch, and want the chest patch to partially cover the tan markings, place the chest patch above the markings in the paint ballz section.
- Problems getting the correct texture? Remember, texture is the 10th/last number in the section. To give it a new texture, like brindle patches, you need to change this number from 0, 1 or whatever it may be, to whatever number the brindle texture has in the texture list. Remember, the top texture is #0, the 2nd is #1 and so on. Count and make sure you've put in the right number.