Object is simple, it's the object's colour. Highlight is the colour applied to the highlight - set the amount of highlight on the slide bar to determine hos much of the object gets highlighted in this colour (true black = no highlight colour), reflection is the colour of the reflects (and can do illogical things) and ambient is strange glow that can make your render awesome, or just be a pain in the arse. Load a simple prop into a new window, set your light map low (256 default is fine) and play around with the colours for a whole afternoon, rendering everytime. There's no better way to get to grips with this stuff.