I guess you're trying to use some official color for the different OSes, but I find the colors for Windows very penetrating compared to the rest, and to simutrans.com pages in general. If there is going to take some time before the Mac build is in place, it might be an idea to give some other visual clues that it is unavailable, such as removing the dummy link and darkening the button. A visitor scanning through the page might read the text on the Windows button (which is in middle and therefore quickly seen), then notice Mac to the right and click without noticing that the text is different.
Hard to tell how the older versions is going to work. I guess it doesn't at the moment because there is no older builds?
There is also no information whether the builds are GDI, SDL1 or SDL2 (or native Mac, but was there ever proper releases of that). I think the old nightlies also had several backends for some OSes, but that's not something for An_dz to worry about unless the build farm is going to make them.