Looks like oil to me. FYI you can cut the end tanks off the radiator if you’re replacing it anyway and inspect the in-radiator oil and transmission coolers to see if they failed. This is the reason some transmission builders won’t warranty their transmissions unless that cooler is bypassed and the only cooler is external. Coolant in your trans will destroy it quickly so hopefully that one isn’t leaking, if you have an internal trans cooler. It’s also the reason I replaced my stock radiator at 150k with a new OEM one. You didn’t mention how many miles are on yours. Follow others suggestions as to draining flushing and filling oil and coolant if you determine that’s indeed what it is. Unless it’s a mix of Dexcool and green antifreeze, you have no recourse at the dealership. It doesn’t look like that to me.