Before you can learn to change your colors one shade at a time, you need to be able to count in hex.
See Learn to count in hex or use the Hex to Decimal Conversion Chart

RGB color notation is one way of specifying what color you want in a particular application, either in a web page, painting program, photo editing program or anything else. Three hex numbers are used, the first one for the red component of the color, the second one for the green and and the third one for the blue.

maximum red(FF) no green(00) no blue(00)	Hex numbers FF 00 00	FF0000
no red(00) maximum green(FF) no blue(FF)	Hex numbers 00 FF 00	00FF00
no red(00) no green(00) maximum blue(FF)	Hex numbers 00 00 FF	0000FF

Different colors are made by combining different amounts of each color component. The higher the hex number for a particular component, the more of that component there is in the resultant color. For example, orange is made by combining a lot of red (hex FF), a middling amount of green (hex 99) and a little bit of blue (hex 33)


For more colors and their hex codes, see the color chart of 216 web-safe colors

The 216 web-safe colors restrict each color component to 6 hex numbers. These web-safe colors can be displayed on any browser. As the browsers get more advanced, many are able to display more colors than just the 216 web-safe colors.
The 216 web-safe colors are various combinations of the following 6 hex numbers:

When you look at the decimal equivalents, you can see that there are actually 51 decimal numbers between each web-safe hex number. These represent 51 shades of color between each web-safe hex color.

From here on, your browser may not be able to see the all the colors. Some browsers are set to change non web-safe colors to the closest web-safe color. I am using Explorer 6 and I can see all the shades. It may not be a good idea to use non-web-safe colors on the web, but for photo editing it could be useful.

Look at what happens if you have only 17 shades between each color instead of 51 shades. I have adjusted the red component by 17 shades at a time from hex 99 to hex AA to hex BB to hex CC. The green and blue components remain unchanged.
The result is four distinct colors

red=99 green=66 blue=CC	9966CC	web-safe color
red=AA green=66 blue=CC	AA66CC	non-web-safe color
red=BB green=66 blue=CC	BB66CC	non-web-safe color
red=CC green=66 blue=CC	CC66CC	web-safe color

Look below at what happens if you have only 1 shade between each color. I have adjusted the red component by 1 shade at a time from hex 99 to hex AA. The green and blue components remain unchanged.

red=99 green=66 blue=CC	9966CC	web-safe color
red=9A green=66 blue=CC	9A66CC	non-web-safe color
red=9B green=66 blue=CC	9B66CC	non-web-safe color
red=9C green=66 blue=CC	9C66CC	non-web-safe color
red=9D green=66 blue=CC	9D66CC	non-web-safe color
red=9E green=66 blue=CC	9E66CC	non-web-safe color
red=9F green=66 blue=CC	9F66CC	non-web-safe color
red=A0 green=66 blue=CC	A066CC	non-web-safe color
red=A1 green=66 blue=CC	A166CC	non-web-safe color
red=A2 green=66 blue=CC	A266CC	non-web-safe color
red=A3 green=66 blue=CC	A366CC	non-web-safe color
red=A4 green=66 blue=CC	A466CC	non-web-safe color
red=A5 green=66 blue=CC	A566CC	non-web-safe color
red=A6 green=66 blue=CC	A666CC	non-web-safe color
red=A7 green=66 blue=CC	A766CC	non-web-safe color
red=A9 green=66 blue=CC	A966CC	non-web-safe color
red=AA green=66 blue=CC	AA66CC	non-web-safe color

This time the difference in the shades is very subtle.

For my own use, I find it sufficient to change each color component by 17 shades at a time.
I use Hex   00, 11, 22, 33, 44, 55, 66, 77, 88, 99, AA, BB, CC, DD, EE, FF
(Bold numbers are web-safe numbers)