I went through the same conundrum with my Volvo D2-55E, they say to replace with the same colour coolant from Volvo, either green or yellow, mine was a light blue colour. After discussions with Volvo I established that the light blue and the green are the same product, it is just that the blue is older and as it turns out the previous owner had decided not to replace the coolant. Not only should you not mix the coolants you should not change from green to yellow without performing an acid wash on the engine as the two are not compatible if mixed, I work in the the industrial diesel engine industry and know from experience that newer engines must have the correct coolant. Newer engines contain more dissimilar metals like copper, brass and aluminium that will be eroded by using the wrong coolant, just like salt water galvanic corrosion does to our under water parts.
Against my better judgement I went for the Volvo ready mixed option rather than the concentrate as mixing it yourself required distilled water rather than tap water, might be more expensive, but a lot cheaper than having to replace the heat exchanger after it has been eroded away.
My D2-55E has the green variety in, replacement interval every two years.
I have attached the Volvo data sheet for the change over from green to yellow coolant.
Hope this helps.