Maybe it's my ignorance, but what has the colour of the cylinder got to do with whether its heating enough to stop legionaires disease?
Most householders can recognise the colour even if they cannot define their plumbing system, so it's an easy question to answer.
OP has not said what colour it is, but the colour usually tells us approximate age and if it is vented or unvented.
An unvented cylinder is fed directly from the cold incoming waterman and unlikely to be contaminated. A cylinder fed with a cold supply will stratify. The hot and cold do not mix to form warm water. The hot water at the top is effectively pasteurised, and being stored, hot, will destroy most microorganisms. The cold water at the bottom is probably uncontaminated, and is also too cold for legionella to thrive and grow, and is replenished with more cold water each time a tap is run.
A vented cylinder, fed from a water tank in the loft, may receive water that is contaminated and/or has been standing for days in a loft heated by the sun and ideal for bacterial and fungal growth. Older systems are more often dirty which provides nutrients for microorganisms.
BTW copper does have some antimicrobial effect, though not total. Stainless can get biofilm, so I guess is not so good in that regard. Neither can be relied upon, so if necessary can be sterilised with heat or chemicals.
Water stored at 60C will kill legionella. The hotter it is the faster it dies. IIRC it dies slowly at 50C, which is OK if you keep the cyIinder hot.
Edit: "The bacteria multiply where temperatures are between 20-45°C and nutrients are available. The bacteria are dormant below 20°C and do not survive above 60°C."
https://www.hse.gov.uk/healthservices/legionella.htm
Hope this helps.