I know this is well after the original post, but in an effort to contribute, I think you can change the font color of the completed milestones using a macro like so:
macro ms_complete [
fontcolor isleaf() & (plan.completed >= 99) "#00D000" # green
]
**you could also apply a background color if you want even more contrast by adding
cellcolor isleaf() & (plan.completed >=99) "#RRGGBB" #whatever color you want
Then in your report you would call the macro in one of the table columns. For example:
columns bsi { title 'WBS' }, name { ${ms_complete} }, start, end, effort, complete,
chart { ...}
I don't know how to change the symbols on the chart. I think you need to do some back-end source code or image library modification for that, but maybe there's a hook somewhere to tweak those symbols within a tji file.