The Castrol Pyroplex EP seems to be a typical lithium complex grease. The lithium complex Mystic JT6 has great specifications and should be equal or better.
I'm no expert on grease, but the water washout test measures just that - how much grease washes away exposed to a spray of water. I don't think it means how well the grease is resistant to absorbing water that oozes into the hub. That's why most marine greases are calcium or aluminum based - they resist water absorption better. You are confusing spray off resistance vs. sponge-like absorbtion resistance. Water washoff would be more relevant to exposed 5th wheels, outdoor excavator/loader pins, etc..
Apparently, your hub company is confident that their design will 1.) prevent water intrusion, and 2.) has a mechanism to flush and replace the grease regularly. Therefore, water absorbing/washout resistance is not critical, just like a normal vehicle bearing. That's why they chose a high quality, high temperature lithium complex bearing grease (tiny trailer wheels spin madly). They don't think they need high water resistance qualities.
Also, FYI, please try to accept that grease color means absolutely nothing regarding quality. Most of us associate red, green, blue as higher quality vs. natural amber/tan grease. You are correct to use the same grease base, i.e. lithium complex, if you don't clean out the old stuff first.
ASTM D1264 Determining The Water Washout Characteristics Of Lubricating Greases
A standard ABEC 6204 test bearing is packed with 4 grams of the grease to be tested. The bearing is rotated at 600 rpm in the water spray chamber at 100°F for one hour. 300 mls of water per minute are sprayed at the bearing assembly. The percent weight loss of the grease carried away with the water is reported. This test is a relative measure of a grease’s ability to resist removal by water.
