The color is used to identify the type. The open-standard color scheme was developed by Opto 22 in 1978 to help identify the four basic types available. The universally accepted colors are:
- Red: Output DC (ODC)
- White: Input DC (IDC)
- Black: Output AC (OAC)
- Yellow: Input AC (IAC)
Sealevel offers a variety of relay racks and solid-state relay modules.