A simple image search tells us which color the front light should look like.
Google Images on Kobo H2O review
The color should be white and not yellowish, redish or blueish.
Off course the white color will vary (slightly) between brands, models and/or individual units.
IMO, the frontlight should be
- white, and
- evenly distributed across the screen, and
- no clouding (especially at screen edges)
If a unit doesn't match the above guidelines it is probably a faulty unit.
It is up to the owner of the unit to judge if the deviations from the standard or norm bothers him or her. Get it either replaced or keep the unit.