Hi Mike and
One thing to consider when using
JPSS on grey is that the window sometimes shows up darker on the gray than other colors. Yellows, light pinks, etc.. they sometimes don't show the window "outline" at all.
The best thing to do is experiment. Remember whereever there is white in your design, it will be the shirt color, and that the shirt color may tint the colors in the design, depending on the depth of each.