Hello K.,
Change just the order:
- color first
- then direction
Here is the expected result:
@startuml
rectangle A
rectangle B
rectangle C
rectangle D
A -[#ff0000]l- B
B -- C
C -r- D
@enduml
[See on PlantUML online server]
or just, without explicit direction indication, but with short arrow and long arrow:
@startuml
rectangle A
rectangle B
rectangle C
rectangle D
B -[#ff0000] A
B -- C
C - D
@enduml
[See on PlantUML...]
If that can help,
Regards,
Th.