This project works really well when looking at colour rather than trying to take on a whole painting/drawing. Get yourself some well known paintings and cut it into sections. A3 works well cut into 8 sections but i have also done some really big jigsaws with over 200 sections as well. Mix up the sections so the children have no idea what the final picture is. This way they just concentrate on the colours and marks in their chosen section.  Give them the corresponding part and they replicate what they see. The images here were all done in watercolour pencils but you can do paints or whatever you prefer. Once complete, connect all the parts and you have a brilliant combined project!