Extender White (aka Ultra Silk Mica) is a blend of Mica and Titanium Dioxide. Its best use is as a"filler" or "extender" for matte cosmetics. It isrelatively translucent and non-whitening and also has a low luster.
If you are making a purely matte product you will want to thin down your intense, pure pigments (such as red oxide, brown oxide, black oxide, ultramarine blue ... ) with something that has good adhesion, relative translucency, and low luster.
In the past we've recommended our custom-blended TKB Matte Texture Base but, to be fair, Extender White is probably a better option.
Generally, you will have about 1/3rd Extender White to pigment. It will lighten the pigment shade.