You can use specify fill and line colors either:

Example Gantt Diagram:

[Activity1] lasts 5 days
[Activity2] lasts 5 days
[Activity1] is colored in White/Red 
[Activity2] is colored in Silver/SeaGreen 
[Activity1] -> [Activity2]

Example Sequence Diagram:

actor Bob #Red/Yellow
actor Alice #FF0000/FFFF00
Alice -> Bob : hello

Also see: skinparam


View colors in PlantUML

A user has recently created an image to display all names colors used by PlantUML. (We thank him by the way!)

So a new feature has been added to print all those colors using a special diagram description:


It is also possible to print a palette of colors close to some other color (using its name or HEX value).

colors chocolate