Custom Art Brush Colour looks dull

My art brushes seem to not have the correct colour.

When saving some shapes as an art brush, the colours always come out as a dull variant of what it was created from. For example, Fig.1 in the image attached is the original art, and Fig.2 is how it appears when used as a brush. Similarly, Fig.3 is the art, and Fig.4 is the brush. The squares beneath serve to give an indication of the colour difference. 

I have no idea what's causing this, could anyone please help?