Microsoft Excel: Conditional Formatting
Colours can be a very effective way of communicating in a spreadsheet. For example you might want to make cells with a number over 100 go red, between 50 and 99 go amber, and below 50 go green.
The Conditional Formatting feature makes this kind of formatting effortless. By changing a number you are also changing it's colour.
It can do way more than that. Learn a few of the options you get with Conditional Formatting then go and play!