Ceiling mount/colour stain

I've seen a few threads in ceiling mount topics mentioning colour stain. Is the method of mounting a factor (i.e cradle or screw-in type mount)? I have a Philips Garbo W/S mounted upside down using a home made bracket and 'hooks' that go into the holes under the pj where the rubber feet prize out. After studying a picture of a mount for it and various other pj's it seems this is how the proper mounts do it.

The pj has only been used for a few hours but there is noticable yellowing especially on the right side of the screen when dark scenes are showing. Is this due to other things maybe, there's plenty of airflow and space all round and none of the mountings appear to be straing the pj's case. I'm wondering if a cradle type may be a better alternative? It has to go upside down for the positioning - but I guess a cradle would put strain on the case the other way?

My idea if I make a different mount would be based on a tv/video wall-mount minus the tv platform. They have the adjustable brackets on the edges which look ideal to hold the pj firmly without it being over tight. I can't just put it on a 'shelf' because of button/remote sensor access.

Anyone with a Garbo that can tell me how theirs is mounted and performing...........??