Blue square versus green squares when sizing labels

[helgas]

While working, I notice the border of the label sometimes changes from green squares to blue squares.  What determine the color of the label's squares please?

[cmatthews]

Generally speaking,

Blue = good position

Green = overlap but if you insist it's okay to have that (some output types may not show the field), PDF is okay with overlapping fields but HTML isn't, stuff like this.

Red = It will not work

This is my own translation but it's relatively accurate.