"I asked this question in the B/W forum and someone kindly replied and told me that with continuous agitation times should be reduced by 10-15%. I just wondered if the same applies to colour developing."

This may be true with B&W chems, but not colour. Your times should be whatever Jobo recommends or 3'15" .

I have been told that mixed chems, exposed to the air will last 1-2weeks on the outside and that mixed chems with an inert gass blanket (as in nitrogen) will last a month (which is what I do with my Wing-Lynch). I would do further research before trusting my opinion.