I had the same thing happen to my '01 YZ426 (with a 444 kit).
The problem is the head gasket.
When problem solving this issue on mine (because it is not normal) I originally thought that I was combining the oil overflow and the water during washing. But I eliminated that by cleaning the tube and running the bike and I still produced a milky residue.
So I replaced my oil thinking that if the waterpump seal was u/s I would have milky oil in the engine, but the oil was good. So I reasoned it could only be the head gasket. I recalled the my cylinder surface had a couple of slight marks on it when I put in the 444 kit. So I stripped it down, I could not see any blow by marks on the cylinder or the head, as evidence of contamination of the oil by the radiator fluid. I was thinking that I was going crazy. I decided to clean up the slight marks on the cylinder anyway. I used a little bit of lapping paste and a glass block (rubbing in a figure 8 pattern) to get the head and the cylinder surfaces flat and clean for the new gasket. Then I replaced the gasket and torqued everything up.
It was all good, the overflow returned to an oily colour and not white anymore. Problem solved.
It is not a big deal and you will have fun seeing inside your new bike. Think of it as getting acquainted.
All the best!