What’s a good color to go with a black and white design, if there will only be one color on the site? I’ve tried using color palettes but they’re not very helpful, because if the first two colors are black and white, the third will be a shade of gray.
What sort of principle should I use to choose a color?
Any colour works with black and white.
Any colour you pick will be effectively an accent colour. This gives you free reign to pick what ever colour will be most effective for the communication of your design intent.
Red is often used with a black and white palette since it is both a bold, powerful colour with a huge number of associated connotations. this makes it very flexible for communicating a number of different things at once dependant on context. Read up on the answers for this question for further information.
Just as important as what colour, is understanding how to powerfully use colour, especially in a black and white design. A ‘black and white + accent colour’ design will find its strength in restaint, it is essentially a minimalist colour palette, and will lose effect if colour is applied too liberally. Careful placement of colour can be used to link elements together, or highlight constrasting elements (or ideas, i.e love and war with red)