I think that you are experimenting with Argentium is great! When Argentium was first being used everyone said they didn't think that it could be enameled. So it is nice to see that it is happening now, I personally don't use it much. The way you are enameling it is very much like the enameling 92.5 sterling. With sterling it is best to raise the fine silver (depletion gilding-getting that nice white) before enameling to get the best results, but that can also yellow sometimes. I would love to see the results from an experiment to see if you applied the clear for silver base coat to the Argentium, if it would change the tone of the colors at all.
With fine silver it is pretty much the transparent colors that yellow, not the opaques, so I can understand why you would not have seen that. If you think you will try more fine silver in the future, you might consider doing some of those great colors samples like the photo above, but two separate ones- one with no base coat and one- with a clear for silver under various transparent colors.
I pointed out the torch firing because the gas can affect the enamel, as you might have noticed when enameling copper. Some transparent but especially with opaque colors, and acetylene is definitely one of the dirtier gases. If you have the opportunity, you might like to try different gas options at some time to see the difference. Natural gas is much cleaner, but you can try propane as well.
Somebody else had asked about counter enameling and I am surprised how many people don't realize that they should counter enamel most of the time. My experience has been that with most cast pieces you can get away without counter enameling, because they are usually pretty thick, and less of the surface is covered with enamel as many people use the Champlevé technique (which has walls) and do not apply the enamel very thick. As you stated you do with your fused Champlevé bases. Formed pieces (especially highly die formed), you can usually get by with one or two thin coats, sometimes none. But, in most cases flat sheet especially, 18 gauge and less should be counter enameled close to equal what is on the front. I have seen thin non-countered enamel pop off years later. I have had many an upset student who forgot to counter enamel or didn't do it early on and the piece cracked. Also, too little enamel with the counter enamel on a thin substrate can easily crack.
If in doubt, 'Counter Enamel'!