Why bright inks look dull on dark shirts, how a white underbase fixes it, why it's printed slightly smaller than your colours, and how to design for dark fabric from the start.
You design a bright orange and blue print, it looks great on screen, and then it comes back on a black shirt looking muddy and dark. This is one of the most common surprises in shirt printing, and it has a simple fix.
Most screen printing inks are a little see-through. On a white shirt that doesn't matter, the white fabric shows through and keeps the colour bright. On a black shirt, the black shows through instead, and every colour gets darker and duller.
The fix is to print a layer of opaque white first, then print the colours on top of it. That white layer is called the underbase.
The printing order looks like this:
The underbase is its own screen, so it counts as an extra colour on your quote. A two colour design on a black shirt is really three screens.
If the white layer is exactly the same size as the colours, any tiny misalignment shows as a thin white line around the edges. To avoid that, printers choke the underbase: shrink it by a hair so it sits just inside the colours. You never see it peek out.
It's a small detail, but it's the difference between a print that looks professional and one that looks home made.
So the underbase question is really just: screen printing, dark shirt, at least one ink that isn't white.
A few habits that make dark shirt prints look better:
Bitgrain's shirt print tool decides which way the ink goes by comparing it with the shirt, so the same look works on a white tee and a black hoodie without you inverting anything.
When you're screen printing a dark garment with any ink that isn't white, you can turn on a white underbase film in Fine tune. It's added to the film set as "Film 1: white underbase, print first", already choked slightly so it hides under the colours, and the print notes tell the shop to flash cure before the colours.
You also see the result on a real photo of the shirt in your chosen colour, so the muddy surprise happens on screen instead of at the shop.
Dark shirts sell well, but they need a little extra planning. Know when you need an underbase, budget for the extra screen, and preview on the actual fabric colour. Try your design on a black tee at bitgrain.app/apparel.