Travellers from European destinations have a green border on their bag tags and may use the blue channel if there is one. Travelling from IAD you will have an all white bag tag. This is a customs identifier and you are not permitted to use the blue channel but must use the green or red where there is one.

So, arriving from the USA you will have white bag tags, clear immigration at LHR then go through flight connections and security. Customs will be at INV, where I dont think there are separate channels, but somewhere there is a red phone that connects you to customs. If you have nothing to declare just collect your bags and walk out. If you have goods to declare pick up the red phone and call customs.

The system is intelligence led, so if you are a person of interest there will be someone there to meet you. If not, then just get on with your day and enjoy Scotland.