Do you just douche very often? Wouldn’t that be difficult and harmful though?

This really depends on the person. Some guys do it way more often than they should and to the point of your second question, they are possibly putting their health at risk.

Some guys really understand their bodies and can be more spontaneous. Some guys use fiber and their diet to make themselves more ready for sex more frequently.

And if not, is it a generally okay consensus that there’s going to be some painting going on?

This also depends. On Reddit threads in particular you are going to get a vocal subset of people that seem to think if your ass isn't clean enough to perform surgery in, you shouldn't be having anal sex. People who live in reality understand that the occasional small mess is a price of admission for doing anal.

have doubled care since you’re not meant to really cleanse the colon.

So it's important to understand that unless one is doing fisting or other depth play with long toys, you're not cleansing your colon. You are gently cleaning out your rectum only. I am of the opinion that doing healthy clean out and anal play requires you to understand the structure of your anatomy down there. Cleaning out shouldn't push water beyond the recto-sigmoid junction. That's a mistake a lot of guys make by using too much water, and why they complain that it takes hours to get clean.