Colour on a poster is powerful, but can be difficult to use well. One of the problems I often see is that people create posters with deep, bright, saturated colours that cover large areas:
It’s hard to get the full effect of this on a small computer screen, but those bright colours on a poster that several feet wide can be like a punch in the eyes. Size signals importance. So do bright colours. The combination of both can be overwhelming and is rarely necessary.
The larger the surface area you colour, the less you have to do for it to “read” as a distinct colour. You can go to a very light, almost pastel shade, and it will still come across as clearly distinct from a background or other colours. People can readily tell that you’ve highlighted something in blue.
If you have something that is very small like a select point of data on a graph, or something for emphasis – that’s the place where you can use those bright, intense colours to draw attention. Subtlety won’t cut it if something is small:
Bright colours can draw attention to something that isn’t dominating by virtue of space.
And the moral of the story is: The bigger the space you colour, the less intense the colour should be. I’m still not sure what the name of this principle should be, but “Inverse colour intensity” will do for now.
(The text? Happy Revenge of the Fifth, everyone!)