The best solution is to print an underbase of white then your yellow on top..That will make your yellow jump off. Or you could print the yellow, flash, print.. and keep printing until you get the results your looking for. Make sure your inks are have cooled between prints or your screen will want to clog on you.. And I guess you could switch to lighter colr shirts.
BTW, I'm assuming your using Plastisol inks..
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