Smartforms are not really great for this kind of artwork, unfortunately. As you noted, there is no option to change color dynamically, so the only thing that comes to mind is a very ugly solution with multiple templates and conditions.
I'm not sure if PDF forms have something more suitable. But why such calendar has to be included in a form? Forms in SAP are meant mostly for rather simple document output. It's hard to imagine a scenario where multiple month calendar would be a "must have" in a business document. Perhaps the users should look for some third party tools if they require such elaborate outputs.