NX698990 11.6.2007
This route is really for biking, but I didn't have my bike with me on this trip so the timings are for walking.
There is a small car park at the end of the minor road (further on from Cairnkinna) just past Polskeoch at NS684021. Follow the Southern Upland Way forest track (ignoring the unnecessary loop to Allan's Cairn) to the 498 spot height at NS697005. There didn't appear to be any option but to follow the SHW SW to the forest edge then S (all on bikeable tracks up until the last bit), then to drop down to the unmarked forest track at NX692990. Leave the bike here and ascend to the trig point on Colt Hill.
The unmarked forest track actually drops down to meet up with a marked one at NX701999 which drops a bit further then ascends to the 498 spot height, so this is a better biking route. I reckon you could do it in under an hour on a bike.
(2 hours 35 mins, 7.3 miles, 1350' of ascent)