Volvo vnl 770 with a dds60 coolent in my overflow tank

So just a quick question to the old mechs or drivers who might have some input on the situation I am in.
So ww williams replaced the head on my Detroit 60 because my old head cracked the injector cups and mixed coolant and fuel after they did that the truck runs like a champ the temp doesn't go above 190-200 even on a hill fully loaded actually it sits between 170 and 190 98% of the time, but I have coolant in my overflow tank it doesn't pour out or leak out the vent hose that comes out the overflow tank but I have never had coolant in my overflow tank before the head swap so I am a bit worried.
Like I said it doesn't loose coolant it goes in the overflow and goes back in the normal tank but what worries me is that there is always about half of the overflow tank full of coolant.
Just need some input at what i could look at on my own

I did a Block test with the little block tester tool that you stick on the coolant filler hole and see if the liquid in it changes color from blue to green or yellow and nothing so I guess the cylinder head gasket is fine. Also changed the air compressor.

Any help would be better then no help Thank you guys

 

At Napa just tell them you need a block tester they know what it is without the part number