Most Oscars can change colour quite dramaticaly - the brown Oscars can go almost grey/orange, and the Tigers grey/gold - usualy triggered by stress - mating - showing off to decide pecking orders - the usual stuff. It only takes them 2 secs to change off - and change back again - but they can hold it for quite a while if they want to.
My Tiger is grey - black - deep orange/red in his natural relaxed state, so for yours to be grey and any shade of red/orange is perfectly normal. There are many variations in adults, and colours can often change a bit as they grow older too
As long as he's not stressed out and your water's ok I wouldn't worry about it.