I don't see the big deal with cowboy boots and half chaps- I have a friend who has been riding with the combination and jumping up to about 3'6 with no issues for the past year while waiting to replace her paddock boots! If you have super loose cowboy boots it might not work, if not then just put them over your current boots. It'll help keep your leg more steady.
Like folks have said, there are synthetic, suede, and smooth leather half chaps. I have an old (~9-10 years) pair of suede Ariat half chaps and love them! Mine have zippers, and I have never had a problem with them slipping. I could see why that would be an issue with the velcro closure ones that used to be so popular around here, but when I looked recently I didn't even see any velcro half chaps that Ariat still manufactured. Smooth leather helps to best create the impression of a tall boot, and from what I have heard from some gives you the best help in holding your leg position.
Buying a quality pair of half chaps may seem like a big investment, but be careful if you decide to go with a cheaper pair! Last I looked the ones similar to those that I have had for 10 years are going for close to $100. At some point in the past couple of years I decided to buy a really cheap pair (~$30) of thin synthetic half chaps in a size a little big to keep lying around if I didn't feel like stuffing my boot cut jeans in my regular ones for a trail ride or something. These things got used about once a month for light trail riding, and maybe 2-3 times per year for a hard ride and they already wore through much of the faux suede covering down to the fabric. They are machine washable, so when they were looking really grungy from sitting around last month I followed the washing instructions and the zipper came off of one of them!
That said, not all synthetic half chaps are poorly made or of bad quality! Just don't think that you can necessarily spend less in the long run by buying a pair.
As far as temperature goes, I would go with something that is non-breathable if you are only getting one pair. I live in south Louisiana and I have no problem with my non-breathable suede half chaps in our hot and humid summers. It is a wonderful feeling to take them off at the end of a ride, but it's not a suffering point when I'm actually on the horse. While our winters don't get as cold as yours, I definitely appreciate the extra layer of warmth in the winter. Having air flow to your legs in frigid, windy weather is probably not what you want
As far as color goes, it's a matter of personal preference. Lots of people say to get black because you can show in them, but at the very low level shows it doesn't really matter. Beyond that level you just need to suck it up and buy a pair of tall boots. Anyway, decide what your personal preference is for half chaps and paddock boots and buy that color. You may be mismatched until you invest in a pair of paddock boots, but when you feel the time is right then buy boots that match your selected half chap color. If you have brown boots and buy half chaps to match, then you'll be running into the same issue if you decide you want black paddock boots but have brown half chaps!! Also, it's worth noting that I ride dressage and have always had my trusty brown half chaps and buy brown paddock boots to match! I showed in them once in my first schooling show before I bought tall boots, and did well!