Instruction 540 instructs the processor 510 to receive measured optical data. The measured optical data may be received, for example, from an optical measuring device comprised in or communicatively coupled to the printing system 500. The measured optical data indicates a measured optical property of a printed test area. The printed test area is formed from a given printing fluid in a set of printing fluids useable by the printing system 500.

Instruction 550 instructs the processor 510 to calibrate the printing system 500 by modifying, based on the optical property of the printed test area, an NPac vector. Modifying the NPac vector comprises decreasing an area coverage in the NPac vector of a first NP. Modifying the NPac vector further comprises increasing an area coverage in the NPac vector of a second NP. The first NP and the second NP each comprise a contribution of a printing fluid in the set of printing fluids. In some examples, the first NP and the second NP each comprise contributions from multiple printing fluids in the set of printing fluids. The first NP and the second NP comprise different contributions of the given printing fluid, namely the printing fluid relating to the measured optical data. The modified NPac may be used in a printing operation involving the printing system 500.