Thank you gents - all of these suggestions could be the cause!! Definitely took a lot of heats for this, compared to tutorials I've seen on line.
Oxidizing flame -- possibly. Yes, it's a propane forge, and I normally just open up the air inlets on the burner pretty wide - maybe too wide.
Colour-wise, yes, I'm red/green deficient but think I can recognise the changes in colour as it cools -- I just don't know what the colours mean! "yellow is mellow, red is dead" is what I had in my head from somewhere on this site.
Thanks