Golf MK4 coolant warning

Hi all,

I've got a 1999 mk 4 golf 1.6S with 50k miles on the clock. Last week, I got a warning "Stop" & "Check engine coolant level". My engine temperature is fine and the car is otherwise running well.

I'm a total beginner with these things but referenced my owner's manual and checked the coolant in the car. The level seemed fine, however the coolant was a thick yellow-brown colour. I released the pressure in the canister and the car stopped showing the warning light for a few miles.

Today I got the same warning again, and opening and closing the coolant tank didn't help. I've seen earlier posts about MK4 coolant issues, but if anyone could answer the following questions I would be very grateful:

1) Does the yellowy colour of my coolant liquid mean it needs changing?
2) If I do need to change coolant, can I buy this over the shelf at a garage or does it need to be changed by a mechanic
3) Is this issue likely to be symptomatic of a sensor error in my car?

Thanks

David