AdiIRC

It would probably not be terribly difficult to implement this, and would be really neat for some channels, but otherwise, some people may not even care.

For Windows 10 Segoe UI Emoji in color, it calls for implementing the correct DirectWrite rendering code, or the hard way, figuring out Microsoft's proprietary color extensions and adding them to FreeType. Sounds more like this project would be taking the DirectWrite path. Supported by more recent versions of Chrome and Firefox, which may be a relevant code reference, if you can dig up the exact change set(s) that made it live in the respective code trees, assuming you're allowed to use open source software as documentation when hand writing your own implementation.

Then there are alternative color fonts, based on OpenType with SVG glyphs. That's more of an open standard, and I'm not even sure if there's a handy .NET code base for implementing that. No Windows systems come with these style of fonts by default, and third party fonts using this tend to be rather rare. Currently only supported by: Mozilla based browsers and mail clients, Microsoft Edge (build 14393+, buggy with strokes in that early a release), and Photoshop CC 2017.