HTML Colors Hex values

Question

I do not get when the hashtag symbol (#) is followed by three hex characters?
And when it is followed by six hex characters?

I Also Would like to know if there is a specific rule for writing the Hex values?

Answer 1

Hex are
Red
Green
Blue

For three digit, each single digit is for one color, from 0 to 15.

For six digit, each two digit is for two color, from 0 to 255.

You don't have to memorizes anything. Use an online HTML color picker, or the color picker built-in in a code editor.

Answer 2

3 hex chars is a shorcut for 6 hex char with twice the same char at each color:

#f00 == #ff0000 == red
#9ac == #99aacc ...

Answer 3

I have a code which converts those values.
https://code.sololearn.com/c4X4zobmnjGa/?ref=app