Hello, thanks, the color is working now. However, a better option would be transparent background. Consider this example:
@startuml
skinparam monochrome true
start
while (<latex>j \leq j_{\max}</latex>) is (<latex>\textrm{true}</latex>)
:test;
endwhile (<latex>\textrm{false}</latex>)
end
@enduml
This produces a png file where the true and false keywords have a gray background and overlay other elements.
Sorry, I cannot find an option to upload an image.