For example: a 1.8m banner with a solid red background and white text. How would a printshop go about printing this? Surely they don't use a liter of red ink.
Screen printing to a vinyl substrate would be your best bet for this size of job.
Let’s say your banner is 1.8m long by 0.6m wide. That gives us a surface area of 1.8m2 or 1,080,000mm2. If the entire banner is printed with red ink using the screen printing process then the thickness of ink might be, say, 15 microns (0.015mm). That gives us a total ink volume of 16,200mm3, or about 0.016 litre.
The screen print process typically lays down a thicker film of ink than other processes, so it is well suited to large solid blocks of colour.
You can go that route, but over-saturation of the substrate is a major issue when blanketing it with that much color. In addition, if the banner is being printed with inkjet, then successfully moving something that large and saturated to the drying rack carries risks as well that all lead back to reprinting. It's not impossible, I've done it lots of times, but it's not easy either.
If the design is simple enough, however, look into a creating a vinyl banner; red canvas with white lettering. Your sign shop will thank you.