I don’t believe there are any color coding standards for seats, but it doesn’t matter. If you are not aware of any standards, chances are many or most of your users aren’t either, so it wouldn’t do any good anyway. Any such color-coding tends to be quite arbitrary, so there aren’t any colors you can choose that, by themselves, users will naturally associate with a meaning.
Instead, rely on pictorial representations in the symbols for the seats. For example:
- First class seats appear visually larger and “cushier” than standard seats (e.g., drawn with convex rather than straight lines).
- Accessible seats may be overlaid with the International Symbol of Access.
- Free seats are empty, reserved appear occupied, unavailable seats have an X through them (or are simply absent).
These pictorial representations should be backed up with text. If there is not enough space to label the seats, then show the text on mouseover and provide a legend for the symbols (and Alt text of course).
That said, you can use color to enhance your pictorial representations:
- Darker and higher saturation colors suggests luxury, so make first class a deeper color.
- Accessibility signs are associated with a specific shade of blue, so use that for accessible seats.
- Assuming you want to direct the users’ attention to available seats, use more muted colors for reserved seats. Gray colors may be best, and may suggest the seat is “disabled” in the UI (i.e., not currently selectable).
- A red X emphasizes “No, stop” more than other colors for unavailable seats (although it could draw unwanted attention to unavailable seats).
All this is more art than science, so it may be worth involving a trained and experienced graphic designer in the project, if you haven't already.
Color is very much a secondary channel of communication in most applications.