@um9s

Peindre

Love the sense of scale in this piece! I think from here there are two things you could do. The first is begin your line art process and eventually colour. The second is go straight into colour and begin painting. Which you choose depends on personal preference and also whether or not you have a good idea of what you're drawing. If you have a good idea and like the look of line art, go with line art. If you don't have a good idea and are going to figure it out as you go, go with painting. I decided on the latter and to start all I did was separate the foreground from the midground from the background into their own layers. Since you've decide on your values already, you can throw on a colour layer over everything and choose your colours. Then you start adding detail. I was too lazy to look up reference of a castle or ruins but I'd highly recommend it so you get something that looks better than whatever I was doing. What colours to add and where all relies on your knowledge of colour theory which you can easily learn just by doing studies of photos. Actually forcing yourself to practice colour-picking without an eyedropper will slowly build up your ability to see colour. Because when we see colour it's not always what we think it is. So practice from reference and it will help you when you try to tackle things from imagination. You could definitely render it way better than I did but I hope this gives you an idea of how to tackle it. I made a video of the process which you can find on youtube /watch?v=IeUViyEk3I8.