Can ya use a pen ? Or colouring pencils when you're drawing ?
I don't see y you would lose marks but I don't think that you would gain any marks either. I wouldn't use up my time colouring the sketch though. You should focus on getting every question done first and looking over them again but the whole exam is very tight on time as well so I don't think you would even have enough time to do something like that. Just be sure that the main sketch stands out with a dark pencil and that the other lines are light but visible.
DylanG i wouldn't use them for projection lines just label them I think you would lose your neatness marks which who count for 14% especially if they smudge and if they cover up pencil work