Abstract: Detection arrangement and method for detecting presence of a residue in a sample by determining color values of the sample, associated with the L*a*b color model, where a value of a composite parameter Z is calculated as follows: Z=wL+waa+wbb where wL, wa, and wb are weighting factors having a value depending on said residue and said sample, and a determination is made whether or not said sample comprises more or less than a predetermined amount of said residue in dependence on said value of said composite parameter Z. In a preferred embodiment, the arrangement is used to detect antibiotic residues, e.g. penicillin-G, in food products, e.g. milk, or body fluids, e.g. blood, urine.