It shouldn't be turning yellow, I don't think. If you've already painted the primer on, its too late to worry about whether it was sanded enough. Did you clean it wellbefore and after sanding? When I painted my bathroom cabinets, I cleaned them thoroughly before sanding. Then, sanded, vacuumed them and then wiped with a microfiber cloth. This was followed by 2 coats primer and 2 coats BM Cabinet Coat paint. My cabinets were dark espresso which I painted white and have no bleed through at all. Do a 2nd coat of primer and hopefully it will take care of it. Be sure to lightly scuff sand between coats too.

Thanks!
I only primed 1 of the bases (a small portion of the entire kitchen.)
I have a lot left to sand and prime.
I did not clean before sanding, only after.
So for the rest of the cabinets I'll clean before sanding, too.
How do I know when I've sanded enough to apply primer?

I went to Home Depot today and a manager told me the primer I used is only good for walls. The said I needed KILZ premium.
So I am going to sand the primer I used and do a second coat with KILZ, I think.

...although I have a toddler and I was going to work after I put her to bed.
I heard the fumes are bad so it looks like I'll have to wait until I can get her out of the house for awhile. At least for the bases. The doors and drawers I can do in the garage.

You are painting kitchen cabinets, so the cleaning prep is critical, you should be cleaning those cabinets with a good degreaser before sanding. You may not notice grease on them, but in kitchens it builds up over time. We refaced our kitchen cabinets 2 yrs ago, the prep is similar to painting. I used Krud Kutter ( a cleaning/degreaser) and I cannot tell you how many times I went over and over the area by the stove. I never saw grease there, I regularly cleaned it, but even so, over time a film builds up. Then, I followed that with a dawn dishwashing detergent and water cleaning, followed with a water rinse and followed that with denatured alcohol. Then, I sanded lightly, vacuumed it well, and went over it again with the denatured alcohol. This was to make sure the veneers would stick (which they are) Maybe it seems like overkill, but if there is any grease or oil, its going to affect the paint adhering. I know its tedious, miserable work, but the prep is so important. With my bathroom cabinets, the cleaning was not as bad, no grease to worry about in a bathroom, but they still needed a good amount of prep. I used Zinsser BIN primer, so I can't comment on the Kilz, perhaps someone can offer their thoughts on that.

Thank you for your feed back. Luckily I started with a desk that is far from the cooking area of the kitchen, and I have only sanded the base so far. I will be sure to clean the rest really well as I move forward. I don't want to take any shortcuts.

You will know you have sanded enough when the surface is smooth and consistent, and you have removed the existing varnish. There is also no reason you can't sand after you have primed, unless of course the primer is still wet. When we do cabinet jobs, we always sand the primer with 220, that is how you best achieve a smooth finish. Before priming we sand with 150.

I assume you are using the Zinsser Bulls Eye 1-2-3 Primer/Sealer, which is a multi-purpose primer intended for pretty much any substrate. That said, I've never used it for a cabinet job, nor would I. It's a water-based primer, which means you would use it under latex or a waterborne alkyd.

We use pre-catalyzed conversion lacquer to prime cabinets, and we usually top coat with a waterborne Alkyd. We always spray cabinet jobs, and we never use latex. It is just not tough enough for kitchen cabinets.

So far I have not found any Kilz primer I would use for cabinets and I personally prefer Zinsser products over Kilz in general, but that's just me.