Yellowing is caused by the flame retardant in the paint reacting to heat and UV light. Therefore if you avoid heat on the surface and use only LED lights then matters not what level of illiminance. Sadly the computer being warm is the biggest issue and also depends how much flame retardant was in the coating. Retrobriting does not eliminate the flame retardant. If you have a machine that after all these years is generally still grey'ish it'll probably stay that way. The keys are the things that suffer most from sunlight.
Anyway, your eyes are way more important than the computer so make sure you have a good level of lighting at all times.
I recently did a series o blogs on the discoloration of retro computers and cus I don't store my machines in boxes and try to keep them cool, dry and out of sunlight they all generally don't fade. If however you stick a computer in a box, in a warm location or in the path of sunlight it'll go yellow.
This is an A600 I bought off Ebay twenty years ago for one British pound and she's still white and has been sitting behind me here all that time. Out of sunlight, LED lamps and light tissue.
https://www.scuzzscink.com/amiga/scuzzblog_august23_1/car_sbd_060823_02.jpgAlso use a light tissue such as kitchen roll over your monitor and computer when not in use to avoid dust entering the casing, keys and ports. The kitchen roll is lightweight, allows the machine to breath and lets the heat dissipate. Anything heavier traps the heat. You are mainly trying to stop dust and insect attack. Spiders love warm computers.
