Anything that strips colour may take out the animal patterns as well. You could, if very careful, use apply Milton with cotton buds just where it has discoloured, be careful though as it can bleed onto where you don't want it to. You could dye the whole thing, but that would amost definitely remove the patterns and probably be in a colour you wouldn't want as it'd have to be dark enough to cover the brown.
If it is as rocky suggests then a mild bleach such as hydrogen peroxide might work, the trick is to use it very weak and soak it for a long time so that you can monitor it and wash out the peroxide when it has removed enough of the wrong colour but not too much of the right colour. If this doesn't work move on to a stronger chlorine bleach. Before you try these methods a soak in dilute vinegar or washing soda (not at the same time ) might do the trick.
If its a baby's comforter, I think that is more important than how it looks so I would do nothing more except make sure that its safe for baby (no residual chemicals). Baby will grow out of needing it and then you can dispose of it.