Abstract: The present invention relates to biomaterials, in particular bacterial cellulose and provides means to prepare pigmented cellulose at acidic pH 5.8, wherein cellulose pellicles comprising tyrosinase (EC 1.14.18.1) are melanated using a development solution at pH 6 to 8.5 and comprises tyrosine, cysteine and/or cystine. Further, the invention relates to means of preparing spatially restricted pigmented cellulose using an optogenetic expression system wherein two polymerase or transcription factor domains are split and each linked to a light-inducible dimerization domain. The invention provides corresponding methods and components.