Reinforced concrete staircase, which covering would you choose?

I would recommend something dull because of the risk of slipping. I would advise against the covering, like RFR in his picture: the reflections alone make the staircase very unsafe because they are confusing and there is no resting point for the eye. Personally, however, I would advise against all dark single-colored surfaces, especially in the staircase area, because dust and hair collect there and the staircase then looks unsightly if it is not walked on daily.

.. and then would something go well with a dark floor?
A light design flooring?