When printing on ash gray or natural shirts you may find that your colors seem a little bit muted as compared to on a white garment.  Unless the image absolutely requires white, you can often get by with printing without white ink, thus increasing productivity and saving both yourself and your customer money.  Here's a little trick - when you go to print select VIVID DARKER under the Color Appearance.

This will over-saturate the image and make up for the color of the garment.  If this does not give you a vivid enough print - increase the saturation of the image in your design software prior to printing.