To add some color in CSS there's a few ways to that:
Starting with background color when added on it makes the background color of that selected item have color.
onto next colour system is RGB and named color which is about when a computer can see over 16mill colors.
An example of RGB is red green and blue characters which each color ranging from 0-255. 255,0,0 would demonstrate max red and 0 to others. This would be used for when balancing out colors.
For Hexidemcimal it's still represented as 0-255 but with Hexidemcimal. With each slot having 16 letters in Hexidecimal. The letters are shown as 10+, eg F=15
A good example is #ffff00= red green blue.
#0f 56 59= red green blue.
Detailed class but important.
Onto the next
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