Osprey's your friend here: Roman Legionary 58 BC-AD 69 (Warrior 71), Roman Legionary AD 69-161 (Warrior 166) and Imperial Roman Legionary AD 161-284 (Warrior 72) are a good primer with lots of pretty pictures. More in-depth and genuine research is provided by Graham Sumner's Roman Military Clothing (1): 100 BC-AD 200 (Men-at-Arms 374). There's all you ever need to come up with colour schemes.
For fancy, if somewhat questionable reconstructions you might take a look at Roman Centurions 31 BC-AD 500: The Classical and Late Empire (Men-at-Arms 479). Only to be taken with more than a grain of salt, and you better forget about d'Amato's so-called analysis.