Are you trying to do some print-on-demand and running into errors regarding ink limitations? Let me help you with that.
Why It Happens
All paper is not created equal. Different papers can only handle so much ink before it becomes smear city. More absorbent paper can handle more ink, but you will be paying for it (much like the difference between one-ply and two-ply toilet paper).
How to Fix It
This solution describes how to adjust your InDesign PDF export options, though it should work with any software that lets you select an ICC profile for export. I'll be using InDesign CC for this example.
First up, download this file and put it somewhere you can find it. Navigate to where you downloaded the file, and right-click (or its iOS equivalent), and you should receive an "Install Profile" option in the menu. Do this thing.
Go ahead and open up your InDesign file, and select your PDF export option of choice.
You'll get an interface screen like this:
Select the "Output" option on the left side. You'll get a screen that looks like this.
What we're gonna do is change a few options here. Check to make sure that under "Color Conversion" that it's "Convert to Destination." We want this. Now, in the "Destination" option, open up the dropdown box and find the "SWOP Coated 240% Ink Limit" profile and select it. Make sure the "Output Intent Profile Name" under "PDF/X" is also the same.
Export as you normally do, but first save your preset so you don't have to worry about this again. You should have separate export options for digital-only PDFs and print-ready PDFs (and if you don't, this is your reminder to set that up). For digital-only PDFs, don't use any color conversion (select under "Color Conversion").
Here's a cover image that has both outputs integrated. Yes, the output is very subtle, but in printing, that's everything.
Shoutout to Legendary Games's "Aethera: Power Armor" for starring in these screenshots.
Profession (Printer) Skill Check
So! What is an ICC profile? "ICC" stands for International Color Consortium, which are industry standards for defining your input (e.g. monitor) and output (e.g. printer). What you see is not what you get, because your monitor is additive color (red, blue, green), while your printer is subtractive (cyan, yellow, magenta, black). If you've ever had a color laser or copier printout and have super-saturated colors compared to what you had on the screen, you're getting a color mismatch. ICC profiles are designed to bridge that gap between what you see and what you get. Most of the time, you will never, ever have to worry about this, because most of the time, you're not exporting out PDFs with super-saturated dark images and backgrounds.
But if you're here, I bet you are. :)
Profession (Artist) Skill Check
Digital art defaults to RGB color spaces. If the piece is intended for print, provide a CMYK option for the publisher to use. If you do the CMYK output yourself, you can make sure the color output is as you intended them to be.