Is wearing a dress and skirt in widely available decorated colors, with a two- or three-colored headscarf, considered a legitimate hijab or an adornment (tabarruj), if the clothing is loose and wide and does not attract attention because men are accustomed to such attire?

There is no harm in wearing a colorful niqab if it is concealing and does not cause temptation. The legal hijab (definition) for women does not stipulate a specific color or form; rather, the important thing is that it be concealing, not describing the body parts, and not inviting to temptation.