The thing is, he had "loyalist" arms, so I retconned him to being "on the turn" to chaos in the Horus Heresy.
I decided to make him a Word Bearer so I went for a modern colour scheme to suit this Legion:
Now the trouble is that when I started painting him I noticed he had some pretty bad mould slippage, which went beyond simple mould lines. You may be able to see some of it on the bolter and bolter arm shoulder plate. I managed to ignore this initially but as the painting went on it started annoying me more and more, until eventually I just rushed to finish (which is probably evident by the colour of the sword!).
Lesson learned - next time check the figure carefully before starting...