Aims:
1. to determine whether mixing 'green' and 'blue' coolant would result in precipitation
2. to determine whether mixing the coolants would alter the boiling point

Methods & materials:

25ml Superplus 3 antifreeze - call it 'A'
25ml Blue OAT-based coolant - call it 'B'
London tap water ;o)

Dilute 'A' and 'B' 50:50 v/v with water. Call it 'C'.

London tap water 'control' - call this 'D'

Coolant mixes heated in 100ml Pyrex™ beaker with blow torch (thought I'd better not use the Smeg™  hob LOL ;o)). When boiling, measure temperature with a mercury thermometer.

Results:

Observations

'A' actually appears yellow, with a green tinge, especially to its meniscus. Good clarity and transparent, albeit with yellow colouration.
'B' was blue. Good clarity and transparent.
When 'A' and 'B' were mixed, 50:50, the result was, unsurprisingly, a bilious green concoction - probably unlike a student's DIY cocktail nasty... Translucent. Looked rather turbid. No precipitates.

Boiling points

'A' boiled at 103.5°C at atmospheric pressure
'B' boiled at 104°C at atmospheric pressure
'C' boiled at 103.5°C at atmospheric pressure
Tap water boiled at 99°C. D'oh. Probably reflects an imperfectly calibrated thermometer - the fact that I was attempting to boil water in a beaker in the garden probably has nothing to do with it ;-)

Summary

Apart from the lovely colour (not), nothing too nasty happened when mixing the coolants in respect to precipitation or boiling point. However, I do wonder if the turbid appearance of 'C' might be reflective of a chemical reaction? Might be worth checking the pH's of each of the coolant mixes. An acidic coolant could corrode the aluminium head...