Compounds useful as hair dyes

A compound of formula I:

• wherein:
• R1 represents C1-10 alkyl substituted by one or more substituents selected from:
• halo, OR2, CN, ═O, ═CH2, NHR2, and C1-3 alkyl, which latter group is unsubstituted or substituted by halo, CN, OR3;
• and
• epoxy, provided that an epoxy substituent is only formed if R1 is C2-10 alkyl and two adjacent carbon atoms of R1 together with an oxygen atom form the epoxy substituent,
• R2 represents H or C1-5 alkyl, which is unsubstituted or substituted by one or more substituents selected from halo, OR4, and CN;
• and
• R3 and R4 each independently represent H or C1-3 alkyl, or a physiologically acceptable salt or solvate thereof.