should i be concerned with fading of a dark color vinyl siding (i have chocolate brown in mind)? in the past i have heard that darker colors will fade over time. is today's vinyl better with color retention? of course i would expect some fading, but would a dark color look bad over the years? i am just beginning to get quotes for the house.

IMHO vinyl siding doesn't look good regardless of the color. I much prefer fiber cement siding but you do have to paint that.

All vinyl will fade over time and it is more apparent in the darker colors. They may have improved the siding with regards to color retention dark colors weren't even an option 15 years ago.

One thing to keep in mind with a darker color it will probably expand more in hot weather than lighter colors as dark colors absorb more heat than light colors. This could compound the "oil can" effect found with vinyl siding, especially siding that is fastened too tightly to the house.

Even a light blue vinyl will fade considerably in a few years. Walk up to the overlaps of any vinyl house and pull the overlap apart.
Vinyl manufacturers are now claiming to have better weathering properties but their warranties read: does not include fading due to normal weather conditions.
Hence, they are telling you they do not fade yet specifically exempt themselves in the warranty from fading.
Go with fiber cement. James Hardie has several dark brown prepainted colors that look phenomenal.