Simple Church Plan (historical)

[atpollard]

OK, that sounds like a worthwhile goal.

First the walls ... the rock probably has a real color (or at least a dominant color) so what if the walls were 'rock' colored ... either outlined in black to make them stand out or all one solid color.

Windows ... for Architecture glass is often rendered blue so many people will instinctively associate blue with sky/open/glass making it a good candidate for 'windows'
Doors ... likewise Brown/wood for common association.
Wall ... the missing wall should be rendered different ... like you have ... but could also be color.

Only trial and error will ultimately show what works with the colors.

Beyond the color, projecting light grey shadows could reveal a lot of details about the windows and roof shape.
It might be worth pursuing to use solid letters like you have for physical items, but label spaces like 'Nave' with an outline font to distinguish spaces from things.

Good luck finding someone with better color vision/sense than me to advise you on actual colors to try.
Arthur