Why did my brown beef stock end up yellow?
The brown color of the stock comes from the cooking method, not the kind of bones, a chicken stock cooked in the "brown stock" method can be as dark as what we expect beef stock to be.

Roasting the bones was a good start; but you also need to brown the vegetables, either with the bones for the last half hour or in a pan on the stovetop.

One of the most important parts of brown stock is to add tomato to the vegetable mix, either finely diced or more often tomato paste. Cooking the tomato to a dark brick red will round out the components needed to make a very dark stock. 

Whether you roast all the ingredients together in the oven or separate the vegetables to the stove; deglazing everything as you did helps the flavor, color, and of course makes cleaning the pans easier :)