No, it's not possible with official release.
However, we have implemented this in last beta http://beta.plantuml.net/plantuml.jar
Project starts the 20th of september 2018
[Prototype design] as [TASK1] lasts 8 days
[TASK1] is colored in Lavender/LightBlue
[Prototype completed] happens at [TASK1]'s end
[Release of new album] happens at the 29th of September 2018
Is this what you are looking for ?