Dark coloured fur is always challenging!
For me it’s a fine line between the actual colour of your pet and what makes
a nice work of art. Too dark, you don’t see the pet.
Too light, it doesn’t look right somehow.

The picture here shows you what I mean (click on the image for a larger view). The reference picture I used is on the left.

She is a pretty little French Bulldog, but very dark. She’s actually a rich chocolate and black mix (brindle).

This portrait was created in ink & wash. The middle image is the first one I did, but as the ink dried I felt it was too light – most clients want the portrait to look like their pet. So, I did it again (right image). This is closer to the picture but I still kept it lighter so you can see the detail in her face and her eyes (artistic license!). But, as always, the quality of your finished portrait depends heavily on the quality (think sharpness and detail) of your original image.