Paraloid B72

I cannot comment on the sandarac as I would advise to not use it. As to a more transparent white, why not use barium sulfate or even a mixture of some titanium white with a chalk or another low refractive index filler which would increase transparency and warm up the color slightly.
I have to admit that I do not understand what is happening to your colors. It is just too difficult to judge this based on photos. Perhaps Koo or another member here can comment.

​It’s a beautifully painted icon, how disappointing it must be for you to see it change.  There are four things I can think of that the bleaching may be: 1. A chemical change in pigments.  2. Trapped moisture.  3.  Semi-transparent, inorganic solids on the surface (scattering light, as Brian noted).  4. Mobile lipids.  I’m not saying these are the only or definitive option (or necessarily even possible!); they’re merely what occur to me. 
It’s often so hard to diagnose what’s gone wrong in a painting because there are so many variables.  Sometimes all it takes is a single, weird anomaly in a material, method or the environment for things to go askew.  So while I don’t think, given your experience, any of the following are likely to apply, on the off chance that they might I’ll mention a few things to consider.
1. Acids can bleach out lapis and ultramarine.  I’m sure you know this, and I understand other colors in the icon are affected (I see the bleaching in the green earth on the side of the nose).  Still, acidity is a consideration.  Did you use an acidic preservative (i.e. vinegar) in the paint?  Or was the paint exposed to acidic vapors of any sort?  (FYI, as an experiment, to see how quickly I could speed up metalpoint tarnishing, I placed a metalpoint test panel over a tray of apple cider vinegar; within 2 days the vapors had completely dissolved every trace of metalpoint – which shows the deleterious effects of acidic vapors!).    
2. When I lived in a state with a lot of limestone, the hard tap water created ‘limescale’ in my pigment pastes; a white, chalky (literally) residue.  If a glaze with a lot of water and limescale in it were applied on a surface, the chalk might appear transparent when saturated; but then, as the water content evaporates and/or a varnish gradually sinks in and sets, the chalky particles could be partially exposed on the surface and lose their transparency.  What sort of water are you using for painting?  If very hard water, best to use distilled.  
3.  As I’m sure you’re aware, do not varnish on humid or rainy days.  The relative humidity (RH) should be approximately 45-55%.  The room should be a moderate temperature, neither too cold nor too hot, c. 65-75° F (18-24° C).  Out of necessity, under a deadline, I’ve varnished slightly outside these variables with success; just be aware that the more you stray from best practices, the more potential there is for problems to occur.   
4. I have this admittedly ill-formed, inarticulate (at this point) idea that polar solvents (such as acetone), which swell the paint film, can sometimes/somehow cause lipids to move about; in short, potentially bring about areas of fatty acid migration.  I know Dr. Stoner has the opposite experience: coatings atop egg tempera suppress fatty acid migration – once the coating has hardened, that is.  But before it hardens, maybe the solvent disrupts relationships among the binder’s components and solids?   Very vague, I know… But perhaps the bleaching is lipids (since FAM on the surface of egg tempera does appear like whitish fuzz), now trapped beneath the varnish.  Unfortunately I can’t clearly explain how or why this might happen. 
5. Varnishes, because they saturate the surface, make everything pop – including visual anomalies.  I understand the bleaching wasn’t there initially, but gradually appeared after varnishing; still, it’s possible the varnish made more visible an anomaly that was to a lesser degree already present in the paint film.  
Finally – although I’m sure this is small comfort – others rarely see a painting as perceptively or critically as the artist who made it.  The first time I saw a live theater performance, I congratulated a cast member only to have him explain, with great angst, all the lines he had missed or fubbed (that I was completely unaware of).  I know the bleaching changed the icon, and of course you want to understand and solve this problem.  Still, the parts of the image you shared are wonderfully painted, and I hope it finds a good home.
Koo​

​It’s mysterious, I can’t explain the bleaching – all my guesses are shots in the dark. I think your approach, trying another solvent, is a good idea.  Also, do get a hygrometer to keep track of humidity; RH effloressence can, in fact, appear inconsistent.  Although it doesn’t sound like that’s your problem, varnishing in high RH is something to be careful of.  

​As for your carbon black, I don’t know why it could be causing the problem but I’ll think about it.  The one notble characteristic of most carbon blacks is that they have very small particle size, are general hard to disperse. ​ But not sure how that relates…

Regarding chalk, I didn’t mean to say you can’t use it; you can.  I only mean to say that, as you know, once the water content in tempera paint evaporates out and the binder settles and cures, pigment particles are no longer full encased by water/binder, and hence go from being fully saturated to being less or unsatruated; hence a pigment’s coloration and degree of transparency/opacity reverts back, more or less, to how it appears in a dry state.  So in a final glaze of a tempera paint with lots of chalk in it, the chalk, which is transparent when fully surrounded by water, may revert to a more opaque state (depending on how encased by binder or varnish it is; the more fully encased, the more it retains transparency; the more exposed to air, the more opaque, white and ‘chalky’ it appears).  Chalk is a fine pigment, it just doesn’t necessarily stay fully transparent in tempera.   ​

Koo