Hello B., and all,
Here is an example using preproc. variable, as:
@startuml
!DateTime="<color:teal>DateTime"
!double="<color:blue>double"
start
:Some block
====
|= field| type|
| StartDate| DateTime|
| Duration| double|;
stop
@enduml

Just avoid to use any PlantUML language keyword...
Enjoy,
Regards,
Th.