While lubing the wheel bearings (Dexter axles) on my trailer, I noticed the old grease that gets pushed out was black on 3 wheels but on the last wheel it came out a light pink color. The grease was a dark red when it was new (Mystick brand). Is the light pink due to excessive water intrusion? Should I be worried?

I have had the same thing happen to me. I would suggest replacing the inner seal on that hub, should stop further intrusion. Good luck!

As for the black grease, do they have brakes on those hubs? When I went through my my trailer hubs this winter, I found the grease was completely black and much thinner on the disc-braked hubs and still the original color on the non-braked hubs. This was after only one season and around 2500 miles of towing at most. The grease didn't appear to be handling the heat well, so I switched to Amsoil synthethic water resistant grease to see if that works better.

Trailer only has brakes on the front axle. They were both black but didn't notice the grease being thinner than normal.
I'm close to the water so I had under 100 miles since the last lube. Probably 10 dips in the lake. Seals and bearings are fairly new and haven't seen a lot of action. I'd understand the black grease after a ton of miles but this seems odd. Think I'll switch grease types/brands and see what happens.

As for the light pink grease, I'll take a look at the seal on that one.

On a side note, how much grease do you pump in when servicing Dexter hubs? I usually pump it in until the color changes back to new color. Took abt 3 full size tubes to do all 4 hubs. Last trailer had bearing buddies and 1 tube lasted a couple of seasons.