OK - I'll give this a try though I was hoping someone else would jump in who is better than I with color aesthetics. There are many factors to be considered if you really want to be proper and follow 'rules', but for practicality sake -
Beginners should not spend a lot on starter pots. Drainage holes are a must. Plastic pots are available in many sizes. However they tend to be very light weighted and can be tipped over easily - especially when the plant material is tall or heavy. Korean 'mica' pots are better if your plant is bigger/heavier. Starter pots should be a bit larger (for 'growing out' purposes) than a more 'finished' pot will be. Over time a smaller pot will be required to present a better proportion or scale. You will often hear that with a 'finished' tree, in some styles, the height of the pot should only be as tall as the thickness of the trunk at it's base.
Other factors to consider - masculine and feminine features of the 'tree'. A masculine tree is one which is usually bold, full, often short, squatty, thick trunked. A more feminine tree then would be leaner, lighter, shapely, more sparse and airy. Pots for the masculine tree should be heavier in appearance, blocky, square or rectangular, darker colored, with simple feet and little or no decoration. Likewise the more feminine tree gets the curves, oval or round pot, more elaborate feet, and decoration.
Pots for evergreen trees, hardy trees should not be glazed - but rather in earth tones, shades of brown to accentuate the colors of the bark. Deciduous trees can be in glazed pots with colors to compliment or accent the foliage - and it gets a bit more complex here. Flowering tropical trees should have colored pots which will work well with the tree while both in bloom or not AND not 'steal the show'. Colors and shades of the flowers and the foliage in Fall can call for nicely colored pots - but -the boldness of the pot should not draw all the attention from the tree but rather accent it/compliment it.
My 'advice' is a bit basic here. There are many variables to be considered. I'm not an authority on these matters but many bonsai books devote chapters on this topic. If your club has a library you will likely find the answers to these questions with a little research.