Q:

Is there a color code for relay housings?

I have noticed that most relays have plastic housings that are white/natural, black, or orange. I have also seen blue, green, and clear housings. I am wondering if there is any meaning associated with this color besides marketing or availability of materials. For example, does it have anything to do with whether or not the device is fully sealed or perhaps its flammability? Or is it arbitrary?

A:

No, there is no standardized meaning of the color of PCB-mount relays. Mostly it's a branding thing- a manufacturer will use a similar color for their line, maybe varying a bit for sealed relays, for example. 
Clear plastic is probably a bit more expensive than colored plastic, but it makes it easier to see the operation or have an internal LED on larger relays. It doesn't look so good on a smaller relay that has been used for a long time because of the contact material that deposits on the inside. And the arcing can cause flashes of light to be visible, which may not be acceptable.
More critically, clear may not be an available practical option if the housing has to withstand SMT reflow soldering conditions. Sealing and solvent resistance can also limit the choices of base plastic. Color and fire retardancy are achieved through additives to the base polymer, which often has a natural color.  

