Not sure here, but I think nicotine when fresh is colorless, but as it oxidizes and is exposed to sunlight it gets darker. I have a bottle of Five Pawns Castle Long that I bought in 0 mg as it was all they had, I added nicotine at home. When it was 0 mg it was colorless but as after I added the nicotine and it steeped for about a week or two it did turn to a light shade of amber. Some manufacturers add colored dyes to the ejuice, which is when you get shades of bright green, blue and red.
Edit: I do have Halo as well and my Kringles Curse 18mg is colorless. But most of the others are darker except TriBeCa which is a light amber. They are all 18mg for those.