"Even whiter than before," vs "even more white than before."

Solution 1:

The comparative form of this word is "Whiter."

Solution 2:

Both forms are grammatically acceptable, with the first used far more frequently (according to ngrams for even whiter,even more white).

Note, some of the instances of even more white linked to at ngrams refer to quantity of white, instead of color saturation or value. For example, in Crows and Jays the P. p. Leucoptera entry says “like last but larger and with even more white in the wing.” This slight difference in meaning and the slight increase in length perhaps account for even whiter occurring more often than even more white.

It is famously easy to write clunky sentences containing the word even, and as mentioned in comments paler or lighter might be better choices than whiter. Perhaps consider

Bathed in the moonlight, her skin looked still more pale.

Solution 3:

Whiter

Yes the answer is "whiter" and it is very easy to remember if you listen to the lyrics of this famous song.

And so it was that later
As the miller told his tale
That her face, at first just ghostly,
Turned a whiter shade of pale"

A Whiter Shade of Pale (Procol Harum) 1967 with lyrics

More white

This collocation is of course possible but not (usually) as a comparative. It can be a noun or part of a noun phrase, e.g.

We've run out of emulsion paint - I'll buy some more white.

This does not mean that the new paint will be whiter than the first batch! It is an abbreviation of "some more of the white emulsion".