i guess you know that a BR5HS is hotter than a BP6HS .. higher numbers are cooler.
A heat range is available because a plug's insulator nose must stay just hot enough to burn away all combustion residues so it doesn't foul .. around 1000 F or so. A cold plug will eventually foul or cause occasional misfires.
But a "cooler" plug doesn't remove any appreciable amount of heat from an engine. The plug itself just runs cooler.
A cooler plug alone won't help cool an overheating engine except in one instance... when the too-hot heat range plug itself is causing pre-ignition.
A nose of a plug who's heat range is too 'hot' will get too damn hot.. maybe 1,200 F or higher. And so the plug itself ignites the mixture before the spark occurs (pre-ignition). This is where a cooler plug is called for. find one that's hot enough to stay clean but not hot enough to cause pre-ignition.
Some tan color is normal on older plugs, but on a brand new plug / plug chop, a clean, white insulator nose indicates you've chosen the proper heat range. That is how you determine heat range.. just pick a plug heat range that stays clean.
Changed weather conditions, different fuel/oil batches, engine modifications or normal engine wear and lots of other things can demand a cooler or hotter plug... so do the normal tuneup maintainance checks, including a plug chop whenever something related to combustion temperatures changes.