How to find what color my roofing is?
Photos can help, but shingle lots vary even within the same brand and color. Your best bet is to remove a shingle and look for a brand. Once you have that there's probably only one color that's an obvious match. 

Either look up manufacturer websites or visit a store with a wide range of offerings and do a direct comparison. Keep in mind that age and normal variation may result in differences. 

That said, do you really want the exact same color for another 20 years? Now's your chance to make a pleasing change, even if small.