fix colouring of notice type message prefixes

previously this was white+bold. the white aspect was dropped since this would not be sensible for users with a white background for their terminal. bold however does not have any effect for me at least so effectively there is no highlighting at all.

here we reintroduce a colour, one that will work for both black and white backgrounds of course. purple looks good to me - significantly different to that used for errors and warnings, and works well with command highlighting (as enabled separately).