|MadSci Network: Chemistry|
Good question! Not being an "ascorbic acid" chemist, I went to the scientific literature to see if I could find out the answer. A cursory scan did not turn up the answer, so I started to dig deeper. The simple answer is that I don't think that anybody is exactly sure what all of the degradation products of ascorbic acid are. There is a lot of literature on the ones that occur as part of the body's metabolic cycle. But in the lab, in water, there isn't a lot of information available.
From what I know, though, yes, the first product would likely be the OXIDATION product, which is the result of the reaction with molecular oxygen (not the reduction of ascorbic acid as you stated in your question). Dehydroascorbic acid, itself, is white to beige in colour and is not likely the source of your discolouration. (I should note that "ascorbic acid" is invariably described as a white to pale yellow powder which would indicate that even in the solid state, there is some discolouration going on.)
However, dehydroascorbic acid is a fairly reactive species and there are a number of other compounds that can readily be formed. More importantly to your question, there are other compounds that can be formed from ascorbic acid that could also react to give coloured products. The diketones could form dimeric compounds, for example. The trick is that you have to have enough conjugation to knock out the blue end of the visible spectrum. In a molecule such as ascorbic acid - or more accurately dehydroascorbic acid - that is not really that hard to do.
Ascorbic acid also has a role in non-enzymatic browning (yellow just being a light form of brown) and there is some work on the capacity of ascorbic acid to inhibit and to contribute to the discoloration of wine to a yellow colour. However, I judge from your question that you don't have anything else in the solution. Non-enzymatic browning requires other components such as sugars.
But to continue with your question, I doubt that hydrogen peroxide is a suitable oxidant in this case. If you would like to explore this further, try something like persulfate or an electrochemical cell or even just bubble through pure oxygen which should result in oxidation of the ascorbic acid.
And with regard to whether or not it is the molecule that turns urine yellow, there are a whole bunch of molecules that do that task, namely urobilogens. Beta- carotenes can also be excreted in the urine to give a dark yellow, almost orangy colour. There are a lot of compounds in urine and you can't really trace the colour back to just a single molecule!
Sorry that this is not much help!
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