Looking for a (non-yellowy) white chalk pencil for marker illustration highlights
I would suggest the Stabilo Carbothello Chalk-Pastel Pencil in Titanium White.

I found one for sale on Amazon for $3.84, here: http://a.co/gimVlHQ

But, I'm sure you can find it in most office supply and/or art supply stores.

There are several other brands, but I would seek out those pencils labeled, "Titanium White" to prevent the yellow-ish tinge you refer to.

Titanium White is known for being bright white. If you're looking for a more transparent, but also clean white, try to find a "Zinc White."