Unfortunately, it is not possible to save custom colours to the colour palette within the email editor. However, you can still use custom HEX colours by typing in the specific colour code in the HEX text box :
- Double click on some text to enter edit mode
- Highlight that text, then press the 'A' button in the blue tool bar, above.
- Type or paste in the hexadecimal colour code in the top right of the dialogue box.
The colour that you use will be saved in the 'Recent Colours' section of the colour picker, so you can use it again without having to type in the code.