The following is from permatex....
color.....................special property......max temp..........applications
Red........................High temp...............650*F....Valve covers, oil pans, timing covers, water pumps, thermostat housings, transmission pans
Blue.....................sensor safe...............500*F...Valve covers, oil pans, timing covers, water pumps, thermostat housings, transmission pans
Orange.......Sensor safe AND High temp....650*F...Valve covers, oil pans, timing covers, water pumps, thermostat housings, transmission pans
Ultra-Black.........Max oil resistance.........500*F....Valve covers, oil pans, intake manifold end seals, timing covers, transmission pans ALSO sensor safe and fast curing
Ultra-blue............Sensor safe................500*F...Valve covers, oil pans, timing covers, transmission pans, differential covers
Ultra-copper.........Max temperature.........700*F... Exhaust manifolds/ headers, valve covers, oil pans, timing covers, water pumps, thermostat housingsAlso sensor safe
Ultra-grey.....High-torque/high-vibration..500*F.....Valve covers, oil pans, intake manifold end seals, timing covers, water pumps, thermostat housingsdoes not say sensor safe
In reality, for 90% of applications, they can be interchanged. But there are "slight" differences and it is alot of marketing to give customers choices.
A few more notes:
all of the "ultra" RTV's are second generation. Supposed to have better flexibility and seal ability. The Non-ultras are first-gen.
All of them except Ultra-copper and Ultra-grey say they can be used on Transmission pans. But I would NEVER use a silicone on a transmission pan. EVER.
Ultra-black and Ultra-blue dont list water-pumps and thermostat housings for an application. So maybe the coolant does something???
Ultra-copper is the only one listed for exhaust manifolds. Allthough I would never use just a silicone for an exhaust manifold.
Red and ultra-grey are the only two not listed as sensor safe.
That about sums it up. So take your pick. I normally used red. No reason at all though. It works on everything I use it on. It isnt sensor safe, but the only place you would use it that would effect the o2 sensor is on the exhaust. And again, I dont use it on the exhaust. Now if you used it like on an oil pan or coolant system, and you "burned" one of those fluids, I suppose it "could" damage the 02, but I think you will have bigger problems to worry about.:laughing: In reality, I dont know if there is really any difference other than color. Maybe it is just to make you buy more sealants??? Cause unless you are in business and use them ALOT, you get one use out of them. Next time you need it, it is usually hardened up.