Blue gray= Ground/foundation quality support. From there up to before red, a piece can be placed. Once its bright red it will demolish within seconds or so after attempted placement.

Initially when placed the piece will check its connected status to others. This is why you see for a few seconds a piece change color.

All in all if you have pieces being demolished, it likely means you're not adequately supporting it. Remember too removing pieces can weak a structure after placement.

Without seeing a detailed example of what you're trying to do, its really hard to discern where your issue is exactly taking place. We often think we have a strong build when in actuality it isn't too so keep that in mind.

Another common issue for some builders is when placing wooden or similar floors that the ground below it is actually not level and the placed piece(without looking carefully) is not bluegrey but actually green. That can then affect pieces placed on or connected to it too.

Lastly, there is for some reason a longer delay in turning red for iron beams placed horizontally too far out that I've noticed in the past. This may be a contributing factor