Colouring illustrations - RGB or CMYK?
I am in the process of colouring a series of hand-drawn illustrations in Photoshop using a graphics tablet. The colouring process takes place mostly using Photoshop's native drawing tools - the pencil, airbrush, etc.
The final goal for these illustrations is printing on a digital offset printing machine.
My plan was to work in RGB colour space to have all of Photoshop's functions available, and worry about CMYK conversion at a very late point - or even sending RGB data to the printer's, letting them take care of the conversion and then fine-tune during the proof printing process.
Are there arguments from the world of professional print production that speak against this? Should I be taking CMYK into consideration much earlier so I don't use areas of the colour space that can never be reproduced on paper?
You don't need to work in CMYK directly. Stay in RGB as long as you want. When you're ready to send to the printer, save your original, then Save As PDF/X-1a using [filename]_CMYK.pdf or something similar, which will convert to CMYK on the fly. Use as your output color profile in the PDF dialog the one that your printer recommends. (ALWAYS ask your printer.)
You can also use
Edit > Convert to Profile and select a CMYK profile. The end result will be exactly the same.
You'll probably lose some color contrast once you convert from RGB to CYMK. Some colors shift slightly depending on the rendering intent & the cmyk color profile you pick. That's why pre-press production (from RGB to CMYK) is a good skill to have. Someone like Cartier-Bresson was an excellent photographer but he had nothing to do with the final quality of his images on film/paper. That quality was created by pre-press operators/lithographers who understood that printing is an art in itself.
So, illustrate/design in RGB. From there decide on the output and convert appropriately. Know how to get the best conversion for those hard to reach - out of gamut - colors. In my experience the printer will just do a quick'and'easy approximation and is not that interested to get the optimal result out of your image. If you want to nail it, you should send a printed color-proof with your digital work. Just don't be disappointed if they have a hard time getting those greys neutral. Some slight changes in humidity might change ink color balance.
If things like UCR, ink coverage, sharpening the cyan channel, paper profiles (coated vs. uncoated), banding, best sharpening method, duotone profiles... don't mean anything to you, here are some resources to get informed; https://www.amazon.com/Photoshop-Color-Correction-Michael-Kieran/dp/0321124014
Also found some good stuff from Dan Margulis.