One of the most difficult things for me to come to grips with was how to paint a white object and still give it form. Like many beginner artists I thought a white object had to be just that "white". I have slowly realised, and learnt to see, that even white objects have a variety of colours in them. It is these colours which help define and emphasise the form of the object, help a cup look round or a face have shadow.
In this old pen and watercolour piece of some goats you can see that I added some cross hatching to the white areas for shading but had no idea of how to use colour.
From there I progressed to doing negative painting and a small amount of shading in grey, as in this duckling painting...
I was still not happy with the results. There had to be more than just grey for shadows. I began to research how other artists tackle this problem, including following some wonderful online "classes" at WetCanvas. I finally started to "see" colour in the white. There is a whole rainbow of colour in there at times. Hopefully the following paintings demonstrate this (to varying degrees of success still) and help you to see the colour.
Painting a black object has similar issues. I was watching a crow on the roof just a couple of days ago and saw that black too is not always all black. There was a lot of highlights and light areas where the sun was shining on his feathers and lots of hints of other colours in there too. I guess this is a whole other project to pursue, especially as I have two black cats at home I want to paint successfully one day.
Then again remember that not all paintings and drawings have to be done in realistic colour, just have some fun with it.