well i think the most flattering lip tones would be to find a shade that is 1 or 2 shades darker than your actual lip color..you can test the shade by putting the lipstick on one lip and if its just 1 or 2 shades deeper than ur other lip then its perfect..or if u are buying at walmart or any store just hold the shade up to ur lip and use your best judgment..but you also might want to go buy the shade of your skin tone...
fair skin tones: nudes in a slightly apricot shade, pinks and light corals look good but stay away from browns if you have a fair skin tone.
medium skin tones: you can go a bit darker and look great in roses, mauves and berries. dont use bright reds, instead use burgundies or something like that.
dark skin tones: deep plum, chocolate or red is good. and women with dark skin can go with either browns or purples..
i hope i helped a little =]
oh yeah..and if you want to go with any red lipstick on certain days..make sure if your a pinkish skin tone to go with more of a cherry red, olive skin tones can go with 'fire-engine' reds, and dark skin looks good with realy deep reds
try lookin for one which is neither too flashy or too boring n go for one which stays on ur lips for a long time :)