Blue means stability and history. Blue means everything is ok. Think of terms like “blue chip”, “blue blood”, “blue shield”, police cars, and what color is the first place ribon – blue! (I’m not sure about “code blue”…let’s forget about that one for a moment.)
Green means action and growth. Something expected happened and you can move onto your next task. Think of ‘green light’ and ‘green grass’.
Red means stop – something unexpected happened.
Yellow means help. Yellow does not indicate any activity or action like green does, and does not mean stop like red. Yellow is informative feedback that provides background and help information, not directives.
Grey is used for information that need not stand out, but provides some value. Typically used for informative or instructive text that does not often change.