The best way, possibly the only way to learn this is to practise, practise, practise.
Where possible I always try to sell a double matt because you can use a thin sliver of a really strong colour around the inside to draw the eye inward to the art. Generally, you then need something more subdued for the top matt. Also, if you want to frame something like an etching with some of the border shown to reveal the signatures and edition numbers you will find that none of your whites will match the paper but put a sliver of colour around first and anything close will work.
Light is another matter. I have two 3' triphosporous flouros hung 1.5 metres above my design table. These are the same "daylight" tubes used in soft furnishing shops and they show colours, contrasts and clashes up with brutal clarity. They also help to pick up any flaws in the artwork so you can point them out and not get blamed for them later.
Generally speaking, repeating the subordinate rather than the dominant colours of the artwork in the matts works best but opposites can also be effective. For instance, yellow or orange "fires up" purple and mauve. Look at a colour wheel and you will find several other good combinations.
A couple of little rhymes may help:
Blue on Blue? No can do. (Go on, try it!

)
White on White? Try all night (ditto) One will always make the other look dirty, yellow etc.

Finally, do not be intimidated by your customers. Sure, they may say "I just want something plain" and they may even be satisfied with a strip of limed pine and a 2" off-white matt but you won't build a good reputation or a successful business that way. Your job is to wake them up to the possibilities and show then something with a bit of WoW!! You want them to go home, hang it on the wall and brag about it to their friends and send them your way.
Properly done, framing is as much an art as painting is.