While not the right colors for the Ultramarine chapter, I think some of us Warhammer nerds can cut you some slack and just say that this awesome looking warrior right here could be part of a descendant chapter of the Ultramarines lol. Also I think there is some fluff that makes notice that certain special squads have different colors then the core of the chapter.
But regardless of all that, this is still a great piece.
Wait, how is it possible to get the colours wrong on something that is limited only by your imagination? could be a successor chapter, only the Gods know how many millions of Ultramarines are out there, but since there are only allowed to have 1000... I'm betting there are alot of pseudo-Ultramarines under different colours and banners.
Those videos really provide an interesting look at your thought-processes. As an artist myself, this makes learning from better artists much easier to tackle that by my earlier method (i.e. hunting down the artists in question, opening their heads with a pizza slicer and devouring the brains to gain their insight). It's much cheaper and less icky, too!
Also, what everyone else said (neat pic, lack of constructive criticism).
It doesnt matter if you got the color wrong or not to me, its your interpretation of something anyways, and therefore you cant really be "wrong".
Its cool to see a hobby I follow presented by an artist I follow, and actually I think its even cooler when a NON-fan does artwork in admiration because not knowing the colors and so forth makes your interpretation so less restricted by standards and patterns that you might unconsciously adhere too if you KNEW the background and so forth.