There are some red windows on the 737 and the CS-300 as well. Why are they painted red?
The aircrafts shown above are test aircraft and the windows are blanked for mounting test instrument. The image below shows the inside of Bombardier C-series test aircraft, which shows the mounting of test equipment.
Image from airwaysnews.com
Also, the screengrab below from this video about C series testing shows the interior of the C series test aircraft with blanked windows with attached instrumentation.
Also, see this thread that discusses about the same thing.