If your inks are turning colors, then my guess is, you are printing on a 50/50 shirt. The plastisol ink is drawing the dyes out of the 50/50 and turning your white a lighter shade of the color of your shirt. If you want to continue using 50/50, you have to use a LB ink (low bleed) made specifically for 50/50 shirts. When printing white on any color, I always put the shirt on the platen and rotate it under the flash unit while I'm placing another shirt on the next platen. By the time the shirt gets back to you (assuming you have a rotating press), the shirt will be cooled down and the heat will have made shrunk the shirt a little (if at all). Then print one pass of white, rotate it under the platen and by the time it gets back to you, print the second pass of white.
This gives you a nice bright white and good durability. As fizz mentioned, you want to make sure your check your cure. I would purchase a Raytek laser heat gun. When you are curing your shirt point the beam on the shirt as it comes out of the tunnel dryer or while it's under the flash unit. Most plastisol inks need to reach 320 degrees. Once it reaches that temperature, it's cured. With a double pass of white, you may want to go a little higher so the heat can penetrate through the ink, (330 - 340).
