I'm lost for real. For 24 hours I've been trying to understand what's going on with my car. Yesterday I was at mechanics, tomorrow I'm going to the new place to do a double check to confirm the diagnose. 

My car is Nissan Qashqai 2012 J10 1.6 dci (diesel) FWD. Bought 3months ago.

Month ago I went to mechanics to change oil and all the filters. After then I started noticing strange water sloshing sounds when accelerate and not always (mechanics told me that it may be coolant) and that was the time period when it started getting colder then when I turned on heating it didn't heat. I went to mechanics and they told me that something wasn't connected properly after changing cabin air filter. The heat still was not warm enough for human and it is also heating when you accelerate.

Yesterday I went to other mechanics. The first thing they did was CO2 leak testing in coolant reservoir. Tester liquid changed from blue to green. They also noticed the cold engine strange sound, told something about the pressure somewhere, the capacity of coolant change in reservoir before and after. I stopped hearing things since they told me that the problem could with the head gasket.

Notice: coolant reservoir is kinda turned into dark colour and the coolant inside is brown (i think because of age).

I started googling a lot and I was trying to search for as many symptoms of head gasket problem as possible. But the only one thing that says it is head gasket problem - the CO2 test. Car drives good. Oil is clean (no milky color). Car is not overheating (the arrow is always pointed to the middle). I don't need to fill coolant. Didn't notice any bubbles in coolant reservoir. Car starts good (even when it is colder right now outside). Haven't seen some kind of white smoke coming out of exhaust.

Overall what do we have? Tomorrow i'm going to another mechanics but I want to get as many opinions as possible. Some agrees that the car needs head gasket replacement, some say that there can be the problem with heater core etc etc. idk