Markus,
the Horsehead is a H alpha emission object.
There are also (in all H alpha nebulae) resonance emissions in H beta ( close to 50% of the H alpha, can be affected by dust etc).
As the eye is more sensitive to the H beta that's why it "seems" to be better seen in H beta rather than H alpha.
I have in my library, what is said to be the definitive work on colour vision:
Researches in colour vision and the trichromatic theory, by Sir William de W. Abney. Longmans, Green and Co, 1913
You may find a copy in archive.org. Here you go:
https://archive.org/details/researchesincolo00abnerich