We can change the line style and color when we "draw" the line in our PlantUML definition. We must set the line style and color between square brackets ([]). We can choose the following line styles: bold, plain, dotted, and dashed. The color is either a color name or a hexadecimal RGB code prefixed with a hash (#).

In the following example activity diagram we apply different styles and colors to the lines:

@startuml

' Make a dashed line, alternative syntax for ..>
(*) -[dashed]-> "Write outline"

' Make line bold and use color name
"Write outline" -[bold,#green]-> "Find example"

' Only change the color with hexadecimal RGB code
"Find example" -[#ff00ff]-> "Write blog"

' Order of line style and color can be reversed
"Write blog" -[#6666ff,dashed]-> "Publish"

' Use dotted line style
"Publish" -[dotted]-> (*)

@enduml

When we generate the activity diagram we see the different line styles and colors:

Written with PlantUML 8051.