Toyota 2WW 2.0 D4D engine coolant brown

[RossTA]

Good morning, 

I have a 2017 Rav-4 with the 2WW BMW N47 engine. 
 

I noticed last night when finishing work that my coolant was a brown colour instead of the blue long life colour and it was half way between max and min. I did have the coolant cap changed about 6 months ago and it was fine then. I did top up my coolant with the official Toyota long life blue coolant with distilled water at a 50/50 ratio. 
 

I have done some research on the internet and I am concerned that my Rav-4 is either: 

1) Leaking its head gasket 

2) The radiator is rusting internally. 
 

The Rav-4 is coming up to 7 years old and has done 66,459 miles and has not had its coolant changed as it is due at 100,00 miles or 10 years. 
 

Can anyone advise me what is causing my coolant to go brown please and if there is an issue would it be an expensive repair? 
 

Many thanks,

Ross.