Do not insert fill color, change the color of the stroke. It unfortunately will have the same brightness level as the original brush definition image had, so black is not recolorable. That's of course for keeping textures.
To make some coloring possible the original base shape is often 50% grey. Stroke color =black leaves it grey.
But you can insert layer effect Color Overlay.It changes also black. Colorize with it.
Color Overlay (surprisingly) doesn't neutralize Bevel&Emboss effect in case you use it for some easy-to-make glosses, so at least something works better than expected. Actually A.D. has numerous such details.
Be sure there's no transparency in the original base image shape area if you do not want transparent strokes.
ADD: Custom vector brushes are based on bitmap images. They can originally be vector shapes which are exported as PNG, the selected shape only, no background!. Only hue and saturation are selectable when you change the stroke color of a painted shape. Applying effect "Color Overlay" is as easy as changing the stroke color. Try it and see. It doesn't affect the brush, only to the shape which is painted.gets the new color.
Apply effect Color Overlay to a group if you want to colorize a bunch of shapes at the same time:
If it happens that you have lost your Effects panel apply View > Studio > Reset Studio.