My 2004 Desert Runner had the same issue. I replaced the valve cover gaskets at about 130,000mi and was taken back a little when I saw the rear passenger side buildup. There are other posts of people having the same issue on CF. I had babied that truck since I bought it with 24k miles. Changed the oil every 5k/6k miles with Valvoline synthetic and didn't drive it super hard. IIRC the consensus on other posts was it isn't a big deal and is related to the oil not getting pumped/moved into that area so it gets a bit of build up. Someone will chime in if I'm wrong I hope.
Hydraulic valve lifters tapping was a fairly common issue on these engines and seemed to be more common on the passenger side. That said, the color is normal for a VG engine and it doesn't seem to cause any problems.