Gold and yellow are two different takes on the 2nd Company uniform, and the idea with the newest Codex was to put a different Captain in charge of the 2nd Company and have the army reflect his take on the Codex-perscribed uniform. Both are technically correct-- in fact, a yellowish tan would probably also be permissable for the 2nd Company by the Codex.
I personally like the newer uniform a bit better-- the brighter blue and yellow seemed a little too happy, for lack of a better descriptor, while the darker blue with gold banding has a more regal appearance more suited to the Astartes, I think.
Keep in mind that Ultramarines just have to be blue-- the trim is purely an indication of their Company. If you can't decide between yellow or gold, you have red, green, and black to choose from as well for the Battle Companies (3rd, 4th, and 5th, respectively); or, you could use Reserve Company colors for a slightly different twist-- the 6th Company is orange, the 7th is violet, the 8th is grey or silver, and the 9th is a deep blue.
