Message: I have all four colors of these shorts [link].  I want more colors but they don't make theme anymore.  If I bought these [link], is there paint or something I could buy to put the design?

Yes. You have to use the right materials, but it can be done. Polyester can be dyed only with a special kind of dye called disperse dye, but you can use this dye to make designs on paper, and then use a hot iron to transfer the designs to your fabric exactly where you want them.

You have two main choices. One is to buy Crayola Fabric Crayons. These look exactly like ordinary wax crayons, but they are made of entirely different materials. (Don't try to use ordinary wax crayons!) These are easy to find and easy to use; the drawback is that the results do look like crayon work, which is sometimes but not always good for a particular design. You will probably be able to buy Fabric Crayons in a local sewing store; you might even find them at a big store like Target or Walmart. See this page for an example and instructions:
Iron-on Fabric Crayons for Synthetic Fibers.

Keep in mind that, like all dyes, the disperse dye is transparent. The original color will show through the designs you make. This means that your swooshes or stripes must be darker in color than the original color of the shorts. You can put black on any color; it will show up best on the lighter colors. You cannot put a lighter color on a dark background. If your shorts are yellow, a blue swoosh will look green. Dyes follow the usual rules of color mixing. The colors of disperse dye seem dull on the paper before you iron them on, but they turn bright on the fabric.

For a crisper, less crayony look, you will have to buy disperse dye powder and mix it with water to make transfer paint, then paint or stencil it onto paper, let it dry, and then iron it onto the fabric where you want it. Learning how to apply the dye evenly may take several tries, so use a polyester rag if you happen to have one, or buy a quarter of a yard of cheap polyester fabric from the fabric store. Of course, you should do this with the crayons, as well. Never expect your very first attempt at dyeing to turn out perfectly. Be sure to prewash anything you are going to apply dyes to.

For general information about using disperse dyes on polyester, see this page: Dyeing Polyester with Disperse Dyes.

To buy disperse dye powder in the US, mail-order from PRO Chemical & Dye in Massachusetts or from Aljo Mfg. in New York. Follow the manufacturers' instructions for dye application. Here are the instructions from PRO Chemical & Dye for Direct Application on Polyester using PROsperse Disperse Dyes. For your purposes, you probably want to order their new line of Transperse Transfer Printing Dyes. Here are the instructions for using them: Transfer Printing on Polyester using Transperse Transfer Printing Dyes. Study the instructions before you order your dyes, so that you do not forget to order any of the helper chemicals that you will need. It will be most convenient and economical to order their "PRO Transperse Transfer Printing Kit", which costs about $13 (plus shipping) and includes everything that you'll need, except for the fabric and the hot iron.

If you want to put a lighter-color design on a dark background, such as red onto black, you will not be able to use dyes to do it. In that case, you will need to cut the design from another piece of fabric (wash it first) and sew it on, using an appliqué stitch.

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