For touch ups you have a few options depending on where they need to be done and what your primer underneath is.
If you want to match the colour since the touch up is in the center of a piece, find a disposable container of some variety and spray some of the spray paint into there until it pools slightly. Before it dries use a soft brush to hit the problem spots.
If you're touching up an edge or a raised section you just found a candidate for some weathering! If it's a pinch point or high friction point like knees, elbows and shins you'll have visible mechanical damage so bring out the blacks for contrast and then bright silvers for scratches. If you want your armour to look old and poorly cared for toss in some rust tones. If it's in recesses you can go the extra mile and make texture paints by adding paint, PVA and sawdust/sand/ballast to make a paste and then jam that grime in there otherwise go for the ink wash method by diluting acrylics or using oil based paints to get grimy colours in deep and wipe away excess.
