I've had good luck with the quicksilver brand color enhancing shampoo.  My paint horse has a white mane that gets very yellow an dirty.  I put the shampoo in his mane and let it sit for a few minutes before washing it out.  It gets his mane very white.

Orvis shampoo works great, one tub lasts forever.

If the bluing shampoos won't work you might try to chemically bleach the hair back to it's normal color and then maintain it with the color shampoos.

Coloring tails is done quite easily, go down to a beauty supply house and ask for helpful tips.  Go easy until you know what her hair can take.  Then deeply condition it so the hair shafts won't become brittle.

You could also just try an at home color kit for light blondes and see what it does.

Horse tail hair can take quite a lot of abuse, much more than human hair.

I've had palominos for many years and have tried about everything, I think.  I usually start with a good horse shampoo, like Corona, to get it clean, then spray on a concoction of 50-50 rubbing alcohol and white vinegar and let it sit for 10 minutes or so - I would recommend putting a flymask on your horse while using this in case he swishes his tail - if he gets in his eyes (or yours) it will burn! .  I rinse it out and re-wash with the bluing shampoo.

But some palominos have naturally blondish, flax or "dirty" tails.  You're not going to get them any brighter unless you chemically process them.

Found another good product to use to make coats glisten and white/cream colored tails clean! Used E-Zall after 10-16 miles of trails for a day for week. Went through mud and muck as Southern MO is getting dumped on by rain not to mention caked on sweat. I really was impressed with the way my mares coat shines like a gold bar and her tail is now pure creamy white. Took 3 seperate applications for this to completely clean her tail, but she is beautiful now.

Gonna use it this weekend before going to a gaited horse show. Sure does make bathing a cinch now. Now let's just hope the rain stops.... otherwise I'm gonnna build an Ark!

I tried the EZ All,and EQ SOlutions before that,on my black/white mare's black/dingy white/beige tail,but never had that much luck with it.

In beauty school,one of the instructors said to get the developer made for the deposit only professional haircolors such as Redken.So being able to purchase that,I purchased the Redken EQ Shades  Processing Solution.It is a low volume peroxide product that just opens up the cuticle of the hair shaft enough for deposit colors,does not bleach.I mixed it with a little mane/tail shampoo and let it sit,then wash with White & Brite Shampoo.It did better than anything I'd tried.Your hairdresser can purchase this for you.