Lightroom Question - Colour Mask

[cambsno]

I have done this before in LR but cant remember how!!!

I have an image where the people overlapped the backdrop. I simply want to 'paint' the background white. How? When I use the brush tool, it only seems to lighten or darken and if I do impose a colour it is transparent and not solid.

 

[JonathanRyan]

See the bit at the bottom of the brush palette where it says "color" (sic). Click the crssed out rectangle next to that.