Underbase gray can help with color and add an extra layer of protection for bleed resistance, especially when printing athletic garments. Putting gray first, followed by white with different halftones will create a great gray study of design, in turn making overlay colors easier to match. This strategy will also help stop dye migration and smooth the surface for subsequent colors.
The carbon in the gray ink acts as a filter to absorb the dye before it reaches the overprint colors. It has fast flashing and excellent fiber matte-down properties. If the design calls for a gray color, this ink can be toned to match and print it as part of the underbase, saving a screen.
Source: Ray Smith