Compositors will eventually be able to render in and output 10-bit deep colors. It would be nice if grim could process screen buffer formats like XBGR2101010 (or even XBGR16161616F) and save them as 16-bit PNG images..
Fortunately, Cairo already supports (can draw in and save) CAIRO_FORMAT_RGB30; since 1.17.2, it even has a CAIRO_FORMAT_RGBA128F format.
Yes. We should probably extend the screencopy protocol to include color management info as well at some point, but that's a separate issue.
Update: grim now uses Pixman instead of Cairo to composite output buffers. The cleanest way for formats like XBGR16161616 and XBGR16161616F to be supported is if Pixman adds support for corresponding buffer formats. There are currently open MRs for the equivalents of ABGR16161616 and ABGR16161616F.