What the color change mean? Is is part of a rebranding, or is it supposed to notify users about some special state of Gmail app?
Is it possible to get back to the original red icon?
According to the Google forum it is a bug that is due to the lab "Unread message icon" being enabled. If you turn off that lab it reverts back to red. I am not sure what else to tell you and this is definitely annoying.
Supposedly the developer of the Lab feature, is now favoring blue for Google Apps (G Suite), since most users of G Suite also have a personal (red) account of Gmail too. https://productforums.google.com/d/msg/apps/ZXJvWINKpwI/DIrg7F3FBAAJ