Not much really, the tree is taking all the nutrients out of the soil - if you go for a walk in a pine forest you will see there is no undergrowth to speak of except where there is a clearing where water and soil might gather. The best way to get colour under such a tree is with raised beds - though they will also dry out quickly as your fir gets to take all there is in there, or pots and troughs. Also with an east facing position, anything there really has got more than a sruggle on its hands - or roots as the case may be. I have a large red sycamore and it is hard enough to get much going under there, but at least it drops its leaves and allows some light and moisture in during the winnter - but not under your pine.
Well planted pots, left there for a few weeks, then rotated with others will do the trick, but you will need to move them out of there after a short while as they will suffer from darkness and cold in that situation. Whatever you put in pots there will need to be well grown before you put it there for a while, well watered and fed before placing, then allowed a period of recuperation afterwards while another goes in its place - tough spot that.