there are no direct colour codes for most colours. the colours themselves are very large numbers ( in the range 0 to 2^24 - hence 24 bit colours - 3 8bit words representing values for each of red, green and blue)
the rgb function mixes red, green and blue to provide a colour, taking values in range 0 to 255 for each of red, green and blue, to return the actual long integer value
so rgb(255,0,0) is red
rgb (0,255,0) is green
rgb (0,0,255) is blue
rgb(255,255,255) is white
rgb(128,128,128) will be a mid grey
full red evaluates as 256-1
full green as 256*256 -1
full blue as 256 * 256 * 256 -1
so something like rgb(128,255,128) will be a green tinted neutral colour. the higher the numbers the paler the colour.
a colour picker chart might show you the values, otherwise its a matter of playing around to find the colour value you want.
pick a colour you like from the palette and see what number it is. there is no direct way to get the rgb values from a colour number, although a bit of maths can calculate them.