Marigold dye has moderate light fastness. Almost every variety of marigold can be used for dyeing. I did read somewhere that maybe it was best to use a darker color of marigold, so I do tend to grow a deeper orange or reddish orange marigold, just in case, but I haven’t tested whether there is a difference or not in the resultant dye bath.
Marigolds give vibrant yellow, orange/yellows and greens with the use of iron. Use a 1/4 tap of iron with marigolds in the dye bath or in an afterbath for a nice olive green. The same iron afterbath can be used to shift many yellow natural dyes to olive green.