Colors

And what does the CAUTION triangle denote? It seems to be telling me what colours NOT to select.  Why would it do this if all the available colours are CMYK?

By @Ray5CB6

There are two "caution" symbols.

• The upper one shows that you have selected a non-printable color. Click it and the indicator will jump to the nearest printable color.
• The lower one shows that you have selected a non-websafe color. This was important in the days when folks had monitors that could show 256 colors, 216 of which were websafe. Click it and the indicator will jump to the nearest printable color.

Why do you think you need to start in CMYK? We should be able to address that issue.

Jane

You'll have to excuse me for being blunt, but it sounds to me like you shouldn't be working in CMYK at all until you get a little more experience. CMYK is for advanced users, there are so many traps to fall into unless you know what you're doing.

First of all, don't use CMYK for inkjet printers! They expect RGB data, and conversion to the actual inks used is performed in the printer driver. CMYK is strictly for commercial offset print.

CMYK is not an abstract "ideal" model like RGB. CMYK is very specifically mapping real physical inks on real physical paper, fed into a real offset press calibrated to a certain standard. These standards vary around the world and for different processes.

So what you have is a long list of CMYK profiles. Until you know which one applies to the actual print process that will be used, you can't really do anything. Each profile characterizes a certain print process.

In other words, CMYK is limited by the real world; how inks actually behave on paper. It's where the rubber hits the tarmac, so to speak. There are limits to how pure colors you can get from printing inks under various circumstances. This is built into the profile.

Each profile also has a built in total ink limit. If you exceed this limit, you get smearing and drying problems.

The colors you use in CMYK mode are CMYK, based upon the CMYK profile you selected when you make the document, converted from RGB or supplied in that color space. The numbers are highly device dependent. The numbers are based on the scale of the CMYK profile. It is what it is. 

When I click to select a red color, the CMYK, Lab, and (associated RGB of your preferred RGB Working Space) are shown, but notice the Out of Gamut warning by the arrow I provided (triangle). That color of red cannot be produced in the current CMYK color space. 

If you click on the triangle, Photoshop will calculate an in-gamut (actual CMYK color) again based on the CMYK profile defining that document to the closest in-gamut color.