I hope this isn't anything major...but when I was about to attempt a thermostat change I popped off the rad cap and was greeted with this... Not sure if it's oil, but if it is how did it get in?
 
 


 
 

Probably not a good sign as it could be head gasket.
Do you have overheating issues?

No over heating issues nor blowing smoke or have any milky substance under the oil filler cap. Though I do have a stuck open thermostat, which temp gauge never goes to half way unless I'm stationary. 
 
I was in the initial stages of changing it but stopped when I saw the contaminated coolant.
 
 

Could just be the result of different types of coolant mixing/reacting with each other over time.
I'd remove the old thermostat and then do a flush of the system. Then fit the new thermostat and see how you go.

Are you losing any oil or water at all?

Doesn't look like I'm losing any. Fluids checked last Saturday, oil dipstick showing full. Coolant not sure how to tell, but if I were to squeeze the hose the coolant would rise up to the filler neck, so I guess its full.
 
Perhaps it's just as MrSky said about the mixing of old and new coolant (I hope). But we'll see what happens when I get it flushed + new coolant and change the thermostat.

im guessing head gasket take out #1 spark plug and shine a light down and hope the top off your piston is not green from coolant.. like mine was :(
 

Mrskylighter
Could just be the result of different types of coolant mixing/reacting with each other over time.
I'd remove the old thermostat and then do a flush of the system. Then fit the new thermostat and see how you go.

I'd agree, I notice it's green. There's a chance red n green could do this if not completely flushed.

If your car is not reaching operating temperature, then what you're seeing is the result of condensation, being formed under the cap because its not being brought up to proper temperature.  It happens on a lot of city cars that barely ever get driven up to temp or for more than at least 20mins.

Yeah it turns out the the thermostat was a 65 degree one. I've replaced it with an oem one at 82degrees, and boy was it a pain in the ass installing.
 
Now I'm replacing the lost coolant from spillage on the thermostat replacement, and that's a pain in the ass as well.
I've got both bleeding points at the front open, but it's only taking like 100ml of coolant at a time, is that right?
 
Should the front radiator bleed cork be dripping as well?