When defining colours within your website pages you will use a code known as a hexadecimal code. Colours are defined by the amount of saturation over three colour channels these are also known as RGB:
Each colour channel has a saturation from 0 to 255.
0 being no saturation and 255 representing full saturation.
A typical colour code being:
Green (0,0, 255)
Once you have chosen the RGB value for your colour you will then need to convert it to hexadecimal which can be done via graphics applications or online calculators.