It depends whether you mean full, bright, vivid red (that means you're about to loose large amounts of grip unless you start cadence braking) or whether it's a mild almost 'cloudy' red, not so bad - you will still have grip.
It affects some cars worse than others and it's also more pronounced on some circuits more than others.
But it's not great, as quite often shortly after the redness appears your car 'washes out' and understeers wide.
There's different ways to overcome this, you can adjust your brakes or you can adjust your driving style...
But, yes, it can be used roughly as a training guide..
Too much speed into a corner, turning too hard or braking too much / hard / late can all result in red smoking tyres..
