Yes, this is a good guide however it is very simple to do it the old fashioned way.
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The basic HTML color code. The way I look at it would be that no color would be black. [000,000,000]
Full color would be white [255,255,255]
the [###,###,###] is the RGB color code (Red, Green, Blue). It is used in programs like Photoshop.
To make this short and simple, take the RGB color code and follow the steps in the example.
Let's say we have a [223,124,58]
We need to take each of those numbers and divide them by 16. I will explain the reason for 16 later.
So, we should get 13.9375, 7.75, 3.625
Then, you put that into a proportion. .9375/10 = x/16 x=15
That would give us 13 15/16 for the first number. We repeat those steps for the next two numbers. 7.75= 7 12/16. 3.625=3 10/16
Now, we have our base 16 handy, right? (0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f)
We could get df7c3a for our HTML color code.
NOW, to answer the question of "Why do we divide by 16?"
I did a bit of thinking in my head and it just hit me. HTML color codes use base 16. 16*16=256. It all makes sense if you just continue looking for simple reasons.