IP Reservations: allow me to set fixed color for clients

The colors of the bar chart representing client requests are currently random. I think it would be an improvement if I could assign a fixed color to each client in its IP reservation record. If no specific color is set then continue using random colors.

With fixed colors one can quicker recognize particular clients and see if they are misbehaving without the need to hover over the bars (which, at least for me, not always works on touch-only devices).