This is a description of basic signal colors, that have widespread conventions among modern HMIs and UIs.
This color is associated with dangerous and extremely critical messages, like emergency problems, destructible (and nonrecoverable) actions, events that require immediate attention and response.
Yellow color is “cautional” color, that is used to attract operator’s attention to something important, but that is not immediately dangerous or critical.
In some systems orange color is used to highlight messages, that have priority between “very dangerous” and “cautonal” — for instance, it can be used to highlight events, that can be become dangerous soon, but don’t require immediate response.
Green is used to inform the user about the positive results or successful status of workflow process. It is “everything is all right here” color.
This color is used to indicate, that the item or action is not accessible or active currently, but it is not mission-critical or is a normal state of the item.
Blue is default color for selection or highlighting “current” item or action, especially if there is an choice. Also it is used to highlight action that has high priority, but doesn’t have any positive or negative connotations.
Violet or pink
These colors are used for “out of scope” messages, that are not related in any manner to current business process or system, but should be presented to the user. They are used because of their rare usage — normally, user interfaces don’t have any violet or pink colors in the palette.
For instance, violet is a typical color for comments overlays above schemes. Also it is widely used to describe dimensions, fonts, and color codes of user interface elements in design specifications.