Please don't pee in the bucket. Acidic additives might speed up the process but, give a darker color that might as well be brown to black paint.
For rust, you can start with rustoleum red oxide primer and put in a few dashes of yellow and green to shift it to an authentic rust color. What is red oxide? Rusted metal/ iron.
I don't know how many people have a tint kit? Other colors of similar paint can do the same thing as tinting for small pieces.
I have made special finishes and colors for even graghics on an expensive motorcycle paint job. You can convert any colorant into a matching medium by putting in the same medium or the catalyst to the final clear coat and nothing will turn on you in mixing mediums. Going backwards from that, you can create some cool one of a kind finishes by intentionally creating reactions between mediums.
Even running out of a small hand mixed batch of paint and then applying another that doesn't match makes it all look more natural.
This new small key board is going to cripple me.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.