I'm afraid you can't even paint a waterproof tent! The paint will peel off of the waterproof surface.
What I recommend that you do is buy some non-waterproofed white ripstop nylon, sew yourself a tent fly, dye it with acid dyes or paint it with a suitable fabric paint (buy a small jar and test it on a leftover scrap of the nylon first), then treat it with a DWR (Durable Water Repellent) finish such as one sold by Nikwax. The white tent will look good with a colorful dyed tent fly. Does the tent already have a detachable fly? If so, you can use it as a pattern in sewing your own.
(I'm not certain that the waterproof finish will keep out all of the transfer dye, if you try ironing the dye onto a waterproofed nylon, unlike other dyes and paints. The DWR finish will probably prevent an even coloration, but it's possible some color could get through. It might be worth a test, if you can find an inconspicuous place to do it.)
On a nylon or polyester fly, if you've sewn it yourself so it's not yet coated with a DWR finish, you can also use transfer printing. The dyes used for transfer printing are actually disperse dyes, for synthetic fibers only; you use them by painting your design onto paper, letting it dry, and then ironing it onto your fabric. One source for transfer dyes in your area is Omega Dyes, which you can order from online. For a small example of the use of transfer dyes, see my page "Iron-on Fabric Crayons for Synthetic Fibers".
For more details on the question of dyeing a tent, see my earlier blog page, "Is there a way to tie dye a tent?".
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