Well, the first 3 things that come to my mind when I saw this were:

1. Add a light to the scene.
-looks like you have light in there already, but it might not be on the brush stroke from the beginning, so you only see it when the light passes by it maybe?

2. You might wanna try turning off Real Lights in the Illumination section of the brush Attribute Editor.

3. If the brush color is textured, check that stroke Color1 value is not black (OR KEYED!)