External lights - shoot at dusk when the light will be closer exposure-wise to the lights of the trees. You'll also get that pretty blue sky.
Inside - use your flash or strobes for fill - lots of hits and misses. Take without adjusting until you get the lights the way you want; tree likely too dark so then add flash/strobes adjusting until you get the right balance. Another way to help out might be to turn on lights in the room to get the right balance of being able to see the tree lights while also getting detail in the tree. Also, make 2 exposures (on tripod of course) - one for shadows and one for highlights and merge in PS.
both - use tungsten or fluorescent white balance