Bicycle Mechanics - How do I determine what size seatpost collar (clamp) I need?

I'm riding a Mercier Kilo TT steel frame that uses a 27.2mm seatpost. I would like to replace my bland silver seatpost collar with a colored one, but I'm confused as to what size I need.

How do I determine? Do collars go by the seatpost size or the frame tubing diameter?:o

How do I determine? Do collars go by the seatpost size or the frame tubing diameter?:o
Whoever makes the collar has no way of knowing the wall thickness of your seat tube, so your seatpost size is irrelevant. The outside diameter of your seat tube where the clamp will go is the dimension that's important. I think some of those clamps also have a lip at the top, slightly smaller, to locate them and keep them from slipping down when loosened.

So do I just buy a set of calipers and measure my outside diameter of the seat tube?

Yes. Digital calipers are dirt cheap nowadays, under $20. They're indispensable for anyone who plays with bikes. However, if you don't want to do that, just measure the circumference of your seat tube with a strip of paper and a tape measure and divide by pi (3.1416) to get the diameter.

does the bike have a front mech?

If its a band on type. take it off and it may say on the inside of it, the diameter.
though it might be a multi fit type. with bits of plastic and steel shims.

I think it'll be a 28.6mm size though

My bike has a 31.8 clamp for a 27.2mm post. I would think that your seattube would be like a soda can to have a 28.6mm seatpost clamp measurement.
Unless you can find specific info for your bike, calipers would be the best bet. I tried to find the info about mine, but I could not, so I got a pair of calipers.

You could always remove the seatpost and seatpost clamp to measure the outside diameter of the seat tube with a ruler. Many of the modern steel frames are using 30mm clamps.

My 2007 Lemond Buenos Aires has a 35mm seatpost clamp for a 27.2mm seatpost.