Image compositing in TileMill by Kim Rees.
From the post:
TileMill is a tool that makes it easy to create interactive maps. Soon they will be adding some new features that will treat maps more like images in terms of modifying the look and feel. This will allow you to apply blending to polygons and GIS data.
BTW, a direct link for TileMill.
On brief glance, the TileMill site is very impressive.
Are you tying topic maps to GIS or other types of maps?