...a melting point greater than 750 degrees Celsius'
Look up the melting point of steel. All that is telling you is that a zinc, lead or aluminium cabinet might not be suitable. Brass would be OK, as would silver.
A business requiring one of these does not have the time, inclination or resources to research the requirements as you are doing and then buy a well made, ordinary cabinet and paint it yellow. Who cares if what is being sold is way overpriced - it is a legitimate business expense, offsettable against tax.
For the petrol cans, it is any escaped vapour that will present a danger. You do not want something that traps this. As you say, they might be better outside, in a well-ventilated area where hot works are prohibited.
To prevent spark ingress, just assess the doors: do they have flanges on all sides? If you can poke a piece of mig wire into the cupboard from the outside, a spark can go through that same gap. If the wire has to do a sharp 90 degree turn to enter, that is much better.
You could also read the COSHH regulations yourself (
https://www.legislation.gov.uk/uksi/2002/2677/contents), as they are horse's mouth. I would guess that the word 'yellow' does not appear in the regulations at all.
The regulations will just talk about risk assessment - a task which you can do yourself rather than purchase an item on which you assume someone else has done the same.
Send a seller of these cabinets an email saying you are considering purchase, that you have read the COSHH regulations and would like to see their risk assessment as it relates to the item to verify for yourself that it is in line with those regs.
