odd number of channels

To answer your last question first, I would say the yellow is equivilant impedance of the red and white of that set.
But I would match the ctr and rears with the same cable you use on the fronts, if at all possible. If not at the very least try to match the fronts and center. You want to maintain the same timbre and voicing across the front. This advice is based upon the assumption that your speakers are of one model line, your amplification is either a multi channel or you are matching brand or model for all channels. And also if you are matching speaker cable as well.

am i missing a common product category of dedicated audio cables marketed with three or five channels? a quick check to kimber and audio adviser web sites' interconnect sections shows stereo pairs marketed, but i didn't notice singles, or sets of three, five or seven.

Your main concern is the center channel. 90% of all important info comes from that speaker. Put your BEST inteconnect there. The surrounds are overstated in many cases for what they need to do. They are the dunce cap wearing fellows in the corners of the room. What is most important BEYOND CABLING, is timing. Set up the delay/timing characteristics correctly for your surrounds in that environment. Sure, having the same set of cables is "ideal". But not really. If you let your paranoia influence reality, then by all means spend alot of money to make you feel better. I do it for other things in life.

Wkraft, there is no missing product category. 4, 5 and 6 channel bundled interconnects exist. I have sets from Harmonic Tech and Crystal Cable. OTOH, this is not an issue since you can use multiples of 2 and 3 channel sets.