Lightroom uses a whole raft of different colour spaces in different circumstances.
The working space in Develop module is ProPhoto RGB with a linear TRC.
Library module uses Adobe RGB (and previews are stored in Adobe RGB). This is to minimise step size with 8-bit previews.
As Andrew says, Melissa RGB is used for histograms (to give a more perceptually uniform histogram).
Web module uses sRGB.
There's no choice about any of these. I think this is an advantage: with no settings to get wrong, it just works. If colours vary between Lightroom and any other software, then almost invariably in my experience Lightroom is right and the other software wrong.