Yellow absolutely does not compare to gold, in future if they don't have what you are looking for then hold onto your money and wait until they do. Gold has metallic particles in it that gives an effect far better than yellow, and since yellow is such a pale colour it has a very low pigment which makes it one of the harder paints to use. Generally darker paints are a lot easier to use because they show their colour on the undercoat better and a lot quicker than brighter colours. If you want a good yellow finish, you are looking at using 4-5 shades from dark brown to light brown before applying the yellow so it gets a better finish.
One thing I will suggest is water down your paints a bit. It looks like you are painting way too thick so the paint is going on 'blobby' in some parts, and obscuring detail in others. It takes longer, but the finish on 2-3 thin coats is a lot better than 1 thick coat. This because a lot easier and less tedious and boring once you start experimenting with highlighting, because all of a sudden you are naturally using 4-5 different coats on the model and will be making it very thin layers as they build on each other, it is more interesting to paint when you can see the detail coming off the model with every stroke. But for now, just use a watered down paint, experiment with the amount of water you want not too much but it needs to spread a bit on its own and not sit in a pile when you put it on your pallete (or jar lid, or plastic plate or whatever you have) if it gets too thin then it will act like a wash and just get in the crevices. If the coat looks a lot darker than the paint in the pot and you don't think its right, do another thin coat.
Do you have a wash or ink? Black ink would be my first preference here, or black wash. If not, get some of oyur black paint and heavily water it down so its like the consistency of milk, then liberally slosh it all over the brown areas and teeth/bone. This will add depth and immediately make it a lot better. Make sure it is completely dry before adding any more paint, it will take a while depending on the weather, then do a light drybrush of the brown on it to bring back the colour and make the higher areas look brighter.
Hope that helps.