Black can be a difficult colour to work with especially if it forms the majority of your army's uniform. You can take the grey, the blue or the off white route to highlighting. Grey is the colour of choice for most people as a highlight to black, usually going with GW's Codex Grey to Fortress Grey; this is known as extreme highlighting.
Extreme highlighting can be very tricky to get use too without making your figure look a mess, however if done successfully can make your black armoured Space Marines look awesome. Below is one of my examples of extreme highlighting.
Although it looks nice, some of the highlights are messy and a little dominating. The trick is too now 'blend' the highlights into the base colour of black. This will create a much smoother transition of highlights. To do this simply use a GW's Badab Black wash and evenly cover the entire figure.
The glazed Space Marine still maintains the strong highlights but now looks less messy, as the highlight transition is not as stark. Don't worry about the glossy look, this will go after varnishing.
The final result smooth looking highlights to black armour, done quickly.
Thanks for reading
DiStudios
The 14th Legion