Can I mix the light green coolant the car came with with a blue one ( Pentofrost NF)?
The car is 2006 S60 2.5T

 

they should mix. its the orange DexCool to avoid. If you have concerns, simply do a drain fill with a fresh water (ie demineralized water etc) flush then fill. Always good to do a flush and fresh fill every 5 years or so.