The shade number 4.2 indicates a level 4 (dark brown) base with a .2 (violet/cool) reflect. On natural dark brown to black hair (levels 2–4), you will see a rich aubergine sheen that catches the light — more of a tonal refresh than a dramatic color change. On medium brown hair (level 5), the plum-burgundy nuance will be more visible. On lighter or previously lightened hair (level 6+), the shade may appear significantly more intense, cooler, and potentially darker than expected because fashion tones deposit more pigment on porous hair.
Gray coverage: permanent fashion shades like this are not formulated for full opaque gray coverage. On hair with a high percentage of white/gray strands, the result will be a translucent, highlighted effect rather than solid coverage. If gray coverage is your priority, a natural series shade (e.g., 4.0 or 5.0) would be more reliable, possibly mixed with a small amount of this fashion tone for reflect.
Heavily lightened or damaged hair: the source notes extra caution. Porous hair grabs pigment faster and unevenly — consider a strand test first and possibly shorten processing time.