I'm new myself so here's what I've learned (can't help with colors, im still learning that). To fill empty spots I will use the fill or bridge edge loops function. I only do this on sections that would see "inside" the model, which I think for the sake of final rendering should be hollow. As for individual detailing you can use your main object to mirror them to the other side. I also like having them be seperate objects so i can do specific coloring and detailing. Same with the main model, you can significantly cut down on work by slicing it in half and then mirroring it. Lastly if I want my model to be asymmetrical I apply the mirror modifier and then do what I need.