AUX and BORDER colour selection in Level editor causing an issue

[AndyHOvine]

I have the AUX colour set to $09.

Drawing with the multicolour free brush, the 8 available colours come out as follows:

The White multicolour comes out as a solid white block instead of white and pink pixels. It appears this is because white and pink are using the same colour index for bits 0-2, the difference being bit 3.

Change aux to $0a the now the red colour is a solid block and the white block is working,

I still don't quite understand what causes the problem which you've partially looked at before. Is it due to the bit pattern or the actual colour on the canvas internally? If the latter, would it help to make the Border and AUX colour RGB values every so slightly different ?