An underbase is a layer of ink (generally white or other light color) that is printed as a “base” on a dark shirt for other colors to sit on. This makes the top colors brighter. It’s a similar concept to painting a base coat of primer under your main paint color.
Because you don’t see the underbase, you might be surprised when you get charged for it. However, it is essential to providing vibrant colors on a dark garment!
Why use an underbase?
The first reason for an underbase is to make colors more vibrant. Although we could print each color on its own, an underbase allows the print to have a bright look yet be soft to the touch because you need less layers of ink. The second reason is color accuracy. Printing on top of a white base will mostly eliminate the possibility of the shirt color affecting the top colors.
Is underbasing required?
You might want a “vintage” look for your design. In that case, an underbase may be skipped even with a dark shirt. Just keep in mind the trade-off: a softer, more lightweight print in exchange for less vibrancy and reduced color accuracy.