I don't know about darker solids vs. tartans, as I am an equal oportunity kilt wearer and mix it up. I wear either tartan or solids depending on how I feel in the morning when I wake up, and how late I am running. If I am running late, I almost always grab a contemporary kilt because the pockets eliminate the need to put on a sporran and I usually do not wear flashes with a contemporary. I do agree however that a darker color does seem more versatile. My heather grey UK Mocker can be worn with anything, formal or casual, but I don't think I could make my tan UK original work as anything other than a casual, or business casual, kilt.

Just my 2 cents!