I only have a design tattooed in outline only at minute, I want to colour it and do black shading..So do I start with lighter colours first then finish with dark shading?

Hello, thank you for your question. 

Generally, on a tattoo you’d begin with the outlining, move on to shading and colour work from darkest to lightest. However, this is the sort of question you’d need to be asking your mentor. If you do not have one, we strongly suggest you get one. 

Here’s an article which might help https://www.barberdts.com/uk/advice-hub/how-to-get-a-tattoo-apprenticeship/