Hey man! It's been a while but I've done the 3D modeling of the marketfort with reference to your sketch. I added some basic lightings and shading but haven't gotten into details in building texture. Thanks again for letting me model this, I had a great time building it. It's damn cool. Cheers!
Hm, I dont know. I think it is mainly practice and learning from other artists, especially old masters. I am a huge fan of impressionism, always loved their colours and read a lot about it. Search for books, blogs featuring art and techniques. Traditional art has made everything before, so use it!