good point... and first we will cover the "no coat" bit.
i live in south india which mostly does not have too cold climes except in hill stations like ooty and kodaikanal, so one can go with just a shirt... and a cravat does come of much help in keeping the neck warm when traveling in a open vehicle at night or on a night train.
to your point about styling, though the best way to mix-n-match the cravats with your shirts and pants would be to lay them out on a bed and think through the combinations, i have understood a few things...
1. our aspiring gentleman wears the belt and the shoes in the same color and never are the belt buckles garish or overly large... this sets a dignified tone to the overall dressage and doesn't create a clash with the rest of the dressing, especially when you are wearing one extra bit than others...
2. since the cravat has to stand out, it must contrast with the shirt and pants... so the below cravat is wrongly matched in color and pattern ( too closely matches the pants color )...
the cravat should have been dark brown or dark blue ( shiny plain or nicely patterned ) both of which go well with the blackish shirt and the cream pants.
and of course, only at being forced to take up physical labor should a cravated get his shirt untucked, otherwise all in.
3. the cravat must at all times be tight and proper, unless the wearer is busy with immediate work... so the below mishap just won't do...
though i must say the cravat color and pattern are good.
4. with a white shirt, many kinds of cravat colors and patterns go easily - red, dark blue, light-brown pattern, mix of light-blue and dark-blue to name some...
here is our hired killer about to fire into some unfortunate watermelons... he wears his with top button done but i would suggest leaving that button undone...
5. a red shirt, which only goes with black pants, will need black cravat ( plain or patterned ).
6. shirts of dark brown or dark green go well with dark brown or black cravats.
7. if you have a big-checked shirt, the cravat should be lighter or darker depending on the shirt color.
8. never wear the shirt and cravat of the same color, even if george clooney tells you to...
in his case, the blue shirt would have gone well with dark brown cravat.
9. never go for the "regimental" type of cravat... it would look quite pretentious outside of a military ceremonial...
---
okay, that is what i remember for now... will continue with jackets later.