Pretty much anything can be painted. However painting vinyl presents a few difficulties. 1st off the vinyl does not provide a good surface in which the paint can stick. So knowing this your prep will require you to roughen up the surface by sanding using a 3M pad.. Those green sheets you find in the sandpaper isle if the store. 2nd you need to be sure prior to the sanding of the surface, which is to remove the glossy finish vinyl tends to have, you need to clean the frames first. A good scrubbing with soap and warm water should do nicely.
Lastly you need to purchase a paint that has the correct properties to pain vinyl. You need to go to a paint store only type of store. Do not waste your time at the big box apron stores. They do not have the necessary knowledge on what you should be using to do this. Many paints require special primers and prep. And going to a paint store they will have exactly what it is that you will need to paint the frames.
The only thing I would interject here is however, Vinyl has a tendency to swell and shrink when heated. If your thinking of painting or coloring the vinyl frames perhaps a darker color, you may cause the windows to bind in the frame as dark colors on vinyl tends to make the material to swell and distort within the frame. So be really sure this is what you want to do. But be prepared to suffer if the windows warp and stop working as a result of the painting of the frames.