You simply can't judge by the colour!

Antifreeze is a whole subject in itself, blue is typically IAT, this does not contain 'silicate' as these are known to break seals used in older engines, the disadvantage is this only has a 2 year life, as a guide line this is for cars manufactured circa pre 1998.

OAT is newer antifreeze, it contains 'silicate' and is typically pink in colour, the benefit of this is it lasts longer, say 5 years, it has nothing to do with being an alloy head, most "modern" cars have alloy heads, or indeed complete alloy engines!

As said though, re mixing and the mixture.

Martin