#FFFFFF is white

They are 3 sets of 2 hex numbers.

#FF0000 is red

#00FF00 is green

#0000FF is blue

(you hear of RGB, Red Green Blue, in that order)

Now if you want a darker color (closer to black) you lower FF to a lower value. A darker blue would for example be #0000AA

if you want a lighter color you increase the other 2 color values. A lighter blue would be #2222FF

If you don't like hex there is rgb(0,0,255) which does exactly the same.

There is also rgba(0,0,255,0-1) with an alpha channel.

You can also put any part of that in a css variable and use it where you like.

body{--foo: 0,0,255,0.5; bar:0,0,100 }

p {background: rgba(var(--foo)) }

div {background: rgba(var(--bar), 0.8) }