For the Inquisitors, use a dark grey base colour highlighted with lighter greys, and do the trim in a dark bronze. When painting Inquisitors, keep a dark and gothic feel to the chatacter. Remember these are characters that spend their lives searching out heretics, non-believers, and Chaos Daemons. I never use bright metallic colours on them when painting the metal trim, as it ruins this effect, if the trim is iron or steel use a dark iron or add black to silver to paint it. If the trim is bronze, use a very dark bronze. this can be obtained in three ways: One, is by first painting the area chocolate brown and then drybrushing brass over it. Or by mixing a tiny bit of dark brown with brass/bronze, or by using a good antique bronze paint over a chocolate brown base. Never use bright colours of any kind on Inquisitors.
For the Grey Knights, there are three accepted colour schemes. One is basically the same as the Inquisitors. The second is a steel base colour with dark bronze trim. The third, and I think the best looking, but the least in character for them, is dark bronze basic colour with steel trim with the base colour highlighted with lighter shades of bronze and gold, and the trim highlighted with Mithril Silver. For the bronze version, base coat / primer coat with a dark brown. For the steel base colour version, base coat / primer coat with black. For the grey base colour version, base coat / primer coat with white (if you use a black base coat, grey will look dirty).