Flat base is a colourless matting additive. Stirred into a gloss or satin paint it lowers the sheen while leaving the colour where it was — the shade stays the same, it simply stops reflecting. It is the answer when the tone you need exists only in a gloss version within a range, or when you want to dial the sheen to a particular point rather than take whatever a varnish gives you.
Add it in small increments and stir thoroughly every time, because the matting agent settles out; too much turns the paint chalky and costs opacity. The alternative route is a matt coat from the varnishes section, which levels the sheen across a whole surface in one pass instead of per colour. The shades you mix it into are in model paints, and the other bench additives sit under modelling chemicals.
