Sonny Boy Havidson
Established
I mostly use yellow-green filters. Things look more natural to me so.
I use either a Hoya orange "G" (actually more of a dark yellow) or a B+W 040 (which really is orange). I tried a red filter but apart from losing some light I could barely tell the difference from the either of the yellow/orange filters on Tri-X so it went back in the box. Both the G and the 040 make a very noticeable difference to outdoor scenes with foliage or blue sky.
While there is a lot of advice floating around about filter factors I have not found it necessary to add more than 1/3 stop with either the G or 040, using cameras with TTL metering (despite what the M6 manual says). After some controlled testing, now I don't bother and just shoot the film "straight up" at its rated ISO.
Filter factors that come in the instructions for filters assume use of a handheld meter, not a TTL meter that is reading through the filters. TTL meters do pretty good with most filters at automatically giving he correct increase, but some colors, like red, need a little more exposure than the TTL meter looking through the filter will say.