Edit: Ok, I just re-thought your comment, and perhaps you mean to 'highlight' the existing road paths if they are also bus-routes. That would be trivial if your roads lines have attribute "bus_route". You can also add the attribute, then in QGIS you can select such paths, edit them and set the attribute as 'true', in TileMill you would then add a style to change color for lines with such attribute. Also, if the streets have names and you have a list of such streets, you can do a CartoCSS selection based on the street names attribute.
You might find that someone has already drawn the bus routes you are interested in:
http://wiki.openstreetmap.org/wiki/Buses
(Then you just have the task of pulling the data out of OSM and translating it into a format that you can work with)