Opaque and sheer hosiery can both be worn day and night, but try matching the weight of the hosiery to the weight of your clothing. For example, opaque or wool-knit tights would look better with a tweed jumper than sheer hosiery. When in doubt, go for dark, opaque styles - they are classic and will make you look slimmer to boot. Nygård offers a fantastic selection of opaque and sheer hosiery in black.
Also, adding shoes in a matching shade will make legs look miles long. As for coloured styles, they may be harder to pull off, but are certainly a way to update your look without the need for extra accessories. Just make sure the colour picks up on, or matches, a colour in your outfit & avoid shortening the legs by wearing shoes of a similar tone.