Conditional formatting colors a cell because of its value, and the color follows the value: change a number and the highlight appears or disappears by itself. That's the difference from filling cells by hand.
Home > Conditional Formatting > Highlight Cells Rules (Alt, H, L, H) has the everyday tests: Greater Than, Less Than, Between, Equal To, Text that Contains and Duplicate Values. Select the range first, pick the test, type the value, and choose a format; Light Red Fill with Dark Red Text is the default.
The value box can point at a cell instead of a number, such as =$H$1, so the threshold can change without editing the rule. Top/Bottom Rules (Alt, H, L, T) highlight the largest or smallest items, or those above or below the average.
Clear Rules removes rules from the selection or the whole sheet.