A few months ago I used Sun In on my blonde hair. I now want to dye it dark brown but I am worried about getting blonde roots and green hair... Any advice please?

Go to a hair stylist. If you cannot afford that go to a beauty supply store and ask one of the hair stylists to recommend I color based on what your current color is. Going darker is always easier than going lighter. Haircolor typically goes on a scale of 1-10 or sometimes 11. 1 being black, 2 being darkest brown, and so on and so forth. 10 or 11 are as blonde as it gets. When you go to dark brown you will need to fill the hair first. The stylist can explain that to you. Basically if you paint a white wall with red you will get pink. You should use demi permanent color because it is less damaging and you can always go lighter easier if you don't use permanent color. They should have swatch books at the beauty supply stores to show you the level you are at. Lets say you are at a 10 and you want to be a 4 (which is a pretty dark brown... depending on your perception of what dark brown is), you would need to probably want to color it with a 7 first (whch is filling it) and then use the 4 afterward. This will help when your hair fades, as it will naturally with the elements, to fade more naturally and not have lighter spots. Make sure to consult a hair colorist at the beauty supply before making your purchase. You may want to call ahead because only some beauty supplies carry hair color. Good luck!