The foreground and background check here should check the .rgba, not the color object:

https://github.com/Tyriar/xterm.js/blob/73cb96ddec39e3531d9b55efde01cd7ba538dff7/addons/xterm-addon-webgl/src/atlas/CharAtlasUtils.ts#L54-L75