This is a question that comes up a lot so we thought it would be best to explain what this is all about, & attempt to specify why an under base is required.
Screen Printing inks aren't very opaque, this is for a number of reasons but mainly so colours can blend & mix when printed, also the ink needs to be reasonably thin so it can be printed through finer mesh for better detail. Due to these factors, when you're printing on dark coloured fabric you will first need a white underbase layer to go down as an "undercoat" before printing the colours. This ensures the print is vibrant on dark shirt colours.
Here's a visual example:
The animated image on the left displays the underbase screen printed with white ink, with the other 3 colours printed on top. The middle image displays the final print with an underbase. The image on the right displays the colours printed without an underbase screen, directly onto the black garment.
The use of an underbase screen isn't restricted only to black tees. It's required when the ink colour is lighter than than the shirt colour on pretty much all shirt colours with the exception of white & light / pastel colours. This can include Heather Grey shirts also if you don't want some of the Heather texture showing through the print, which isn't always desirable.
What about white ink? Well, if your design is white ink only, we charge for 1 colour. This is because we can often use a special discharge ink which can be printed with 1 layer. This ink cannot be used with overprinted colours as the top colours won't adhere properly to the ink.