yep there is a high and low intensity palette. the two main are the high intensity 0 and 1, with 0 and 1 being the darker versions
The full CGA color palette is not allowed
So I would have thought FFAAFF was valid.
I think it is the spirit of CGA palette that needs to be kept. So you have a 'pink/magenta' a darker version of that for the other palette, etc