Change color of non-printing characters

I find the + & - markers, that indicate that the duration of a measure differs from the time signature, are somewhat too subtle a pale grey colour, and can easily hide amongst other elements in the bar. (See the last bar of the first line of the attached image.)
Can the color of these non-printing characters be changed?
I have hunted about in the Style options and Preferences, but haven't spotted any item that seems to have an appropriate name!
(These bar-duration errors have come about as a result of converting existing pdfs using the online application. The application is amazingly good (well done!), but does have difficulty, for example, with grace notes, hence the errors in bar length.)
As is often the case, it may be that I don't know what to call what I am looking for! (I have difficulty with using "measure" for bar etc - UK amateur musician!)