[kiririn]

Do any exist with warm colour temperatures? My kindle paperwhite is at least 6500k and basically unusable without an external light source

[terramex]

Kindle Paperwhite 5th gen has both amber and white LEDs and its color temperature can be adjusted. I don't know exactly what light temperature range it has but it feels very "warm" on maximum setting.

[obahareth]

Some of the ones from Onyx Boox have customizable cold/warm temperature sliders.

[tlavoie]

My Kobo Libra H2O has quite a warm backlight, with variable brightness. I don't recall if the colour temperature is adjustable however.