The letters (a,b,c,d,e,f) & numbers (0,1,2,3,4,5,6,7,8,9) in the HTML color tags represent the hexadecimal red - green - blue values of the color you want for. With the first two characters the red value, the second, the green, & the third, blue.

The first table shows a few of the colors that are supported by name too in the browsers, you can use "black" instead of #000000 for example.
(Please note though that different browsers have different opinions on what color a specific colorname really represents, so use the hexadecimal values if you want to be sure that a color comes out correctly in all browsers.)

Step 1 : Get RGB Value

Blue: RGB: 0,0,255: no red (0), no green (0), & full blue (255)

Step 2 : Divide by 16 and multiply decimal remainder by 16

Red Value: 0 / 16 = 0
0 * 16 = 0

Green Value: 0 / 16 = 0
0 * 16 = 0

Blue Value: 255 / 16 = 15.9375
.9375 * 16 = 15

That gives us 0,0 - 0,0 - 15,15

Step 3 : Convert to hexadecimal

Number	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Hexadecimal	0	1	2	3	4	5	6	7	8	9	a	b	c	d	e	f

Result

0 & 0 = 00 - red ("#00 ")
0 & 0 = 00 - green (#0000 ")
15 & 15 = ff - blue (#0000ff")

The result for blue is "#0000ff" as you can see.