What color format should be used to create a logo: CMYK or RGB? (It would be used for both printing and for screen)
You should supply both.
Ideally you should supply screen and print versions in RGB and CMYK and in vector and bitmap formats.
It completely depends on the situation and requirements but an example of formats to be delivered could be something like;
- JPG (RGB - high res & low res)
- PNG (RGB with transparency - high res & low res)
- SVG (RGB)
- PDF - (CMYK)
- EPS - (CMYK)
- AI - (CMYK)
There will normally be variation as well (e.g. Full color, black & white, with/without text etc) and there could be spot colors to take in to account and any specific format requests.