Public transportation lines (metro, subway, underground, MRT) and colors

Of cause there are OSM based apps (examples) exactly answering your question. People are extensively using OSM outside the popular OSM public layers, even if we don't know about them (and that is the nice and amazing fact with OSM). Though, there are two general problems related to your questions:
-The physical T-lines are in OSM but the logical (the logistics) are not. The logical lines are often changing, are specific for cities, varying in colours, colour shades and so on. Therefor, as a rule, these coloured and geo-positioned logical lines, together with the transport dynamics, are application additions to OSM.
-On certain physical lines/segments several logical lines may be positioned (with different colours). Rendering these overlapping cases may be challenging. Instead of drawing parallel lines over the map, drawing a (moderate) thick line with along-the-line styling might be a nice aesthetic replacement/solution.
An example of a similar app is the "UK TravelOptons" from the App Store in iTunes. This shows the London coloured T-lines over the OSM map together with the real-time traffic dynamics.

The colour tag can be used to specify the colour of public transport line in a schematic (http://taginfo.openstreetmap.org/keys/colour#overview) although that would be on the relation, not on a node.

I don't know of a map that uses this though, see also "You can add a color to the transport map?".