I made some blue BC icing (Indydebi's) 2 weeks ao and had leftovers and so i put it in a ziploc bag in the freezer...and now i have to make a cake for someone and will need a darker shade of blue...can i just take that out...and add more food colouring to it...possibly add even more white BC because i will need a bigger batch?

As long as it doesn't taste like freezer, it will be fine to use. I've kept it in the fridge a few weeks even and then used it and it was still great.