I’m swapping out a single light switch for a smart switch. The wiring from the gang is a single cable with red, black, ground, white. The neutral wire was unused in the original switch. The smart switch has the same wiring configuration and the instructions say to match colors. On the others I have installed (same switch same mfg) everything works fine but when I match the colors and connect it all I get nothing. I’m wondering if the neutral wire is not connected somewhere down the line like in the light itself. All other components work fine and the single pole switch works when reinstalled
Red and black are not always going to match up like that. I'm pretty surprised the instructions even mention matching colors. There are conventions, but it's not very reliable in practice other than white always being neutral and green/bare being ground.
You need to read the instructions on your smart switch, and you will find that red or black is going to be for power (line) and the other is going to be for the light (load). Since it's not working the way you have it hooked up, I would assume red and black need to be swapped, but before you ruin the switch, read the instructions and determine if the red or black wire in the wall are hot (line voltage).
More explanation: On a regular switch, what wires are hooked up to the two screws doesn't matter because it's a simple contact closing and the electricity will flow either way. On the smart switch, it needs power on a particular wire to control the electronics. So, the hot wire from the wall must be connected to that particular wire on the switch and the load (light) gets hooked to the other wire.
It looks like you have mixed up the wires. You haven't mentioned which wires were initially connected to your switch. And a clear photo of the smart switch is needed to see which wires should be connected there. You almost never need an earth wire to be hooked up to a smart switch. Just remove that one. You only need to connect 3 wires to your smart switch in this configuration. Live and neutral as the input and just one output wire.