The road and tram track tiles are overlays. The groundtile is not replaced. It is overlayed by the road or tram track sprite. Replace the grass part of Andrew's and Foobar's sprites with transparent blue. This has the added advantage in that you will not need an alternate set of sprites for other climates.
Not exactly. It sounds like SimYouLater is trying to do road graphics "the old way", i.e. not
using NRT, in which case the above isn't true.
If these graphics are indeed meant to just replace the baseset sprites then they will need to include the full tile, otherwise you will end up with weird graphical artifacts in the gaps. One possible option for this would be to modify the OpenGFX sprites to blend in a little better to the TTD landscape. It wouldn't be perfect, but with a little work you could probably get a decent looking result, and it would avoid having to include any sprites that might violate copyrights
In fact, there can't be old without new, and there's no such thing as "new way" in NRT...
Everyone is coming up and say "This, is a revolution". It is, but NRT is not stable yet and will not until a few months, or even a year. Until those days we must do it this way. Why ? Because JGR, Spring 2013 etc, and simply no other patchpack, don't support NRT yet.