Use the intuitive Colour Picker whenever there is an option to change the colour, from font colour to background colour.
If you already have a standard colour, enter the colour code in the Hex box. A slightly less dark black is optimal for reading text in white backgrounds. A good example is #4a4a4a. You can also go ahead and enter the colour in RGB format if that's the method you prefer.
The colour picker will save 16 of your recently used colours for each Email. Every new Email will have no recently used colours, but the colours will remain for each Email whenever you go back into editing.