Simple question: I have to update a TRectangle fill color into a loop, it only works if I set Application.

Simple question: I have to update a TRectangle fill color into a loop, it only works if I set Application.ProcessMessages, is there any kosher way to do that? I tried BeginScene and other tricks but I was unable so far, maybe I am doing the wrong way. Code should run in FMX for both Windows and Android.