All you really need to get your head around is Hue and what it represents. Every 30degrees (value) is a new transition to a primary/tertiary colour. Every 60 degrees is a primary, and every 30 inbetween is a seconary.
See the Hue values below, showing
- top - all of the primaries and tertiaries
- middle - primaries only (every 6o deg) and
- bottom - tertiary only (every other 30deg)
The beauty of it though is that most website colour palettes are based on a distinct set of hues, where these are then modified in saturation and lightness to get differnent shades of the same hues. Defining these in RBG is... well see the example below and you'll see.
In the HSL column, the Hue changes slightly, but right away you can see that the hue is pretty much the same for the slate colours. In RGB, you have to mentally mix R G B values in HEX together to try to work it out, for as many colours as you have -- you may as well just call them "colour1", "colour2", ... "colourN" 
. If you wanted to add a new variation, good luck working it out yourself! And if you want a colour that is slightly less saturated, or lighter, or darker, you must use a colour picker to do it (and you'll be most likely be using the hsl sliders when you do!) else you'll end up with a different hue and lightness