MapManager allows you to select a colour by using the Colour Selector combo box.
By clicking on the button at the right, the standard colour selector dialogue appears which provides selecting from 48 predefined and 16 custom colours according to the figure below:
You can define custom colours by clicking on the Define Custom Colours button, and set the RGB and HSL values explicitly or click on the colour scale in the following dialogue:
Note: By removing the text in the colour selector combo-box a MapServer 'empty colour' is specified. This is the equivalent to "no colour" or transparency.