Maybe I should invent a programming language in which color has semantic
value. And fonts. :-P
Why stop here? Go paint your programs in Piet. ;)
Fixing const issues in Phobos would be as simple as
selecting the entire source code and changing the font to italics. :-P
I suggest a special font designed just for D. ;)
Bold for const, Black for immutable, UltraLight for volatile, keywords
written in Small Caps, comments in Script (must look handwritten)...
Colours, as you mention would replace safe, pure, nothrow.
Of course a special obligatory colour palette will be created.
And as we're at it, it's time to go one step further in creating code
friendlier to read, and get those APL keyboards back from the attic, so
they can be useful once again. :D