Frozen breastmilk, Combining breastmilk and using early milk

So I'm going back to work in a month I already have a pretty good freezer stash. I do have some that is yellow that I have pumped in the early days when my milk first came in and I have some that is just white. Can I still use the yellow? I'm thinking it has some colostrum and thats the reason for the color compared to the recent milk. And whats the role on combining frozen milk? Can u combind the early milk with the recent or does it have to be all from the same date? For example can I combined milk from beginning of January with some from the end of January or February. Can u mix frozen yellow milk with the frozen white milk, is another way of wording it.