•Adulteration in ketchup by adding illegal Sudan I dye.•An eco-friendly methodology based on colour histograms is proposed.•Multivariate analysis to identify and quantify Sudan I adulteration.•SPA-LDA classified correctly all the samples in both training and test sets.•iSPA-PLS achieved good performance to quantify Sudan I adulteration. Sudan I is a synthetic-azo dye commonly used to adulterate foods to increase sensory appearance. However, it is banned due to its carcinogenic, mutagenic and genotoxic properties, which represent a serious risk to human health. Thus, this paper proposes a feasibility study to identify and quantify Sudan I dye in ketchup samples using colour histograms (obtained from digital images) and multivariate analysis. The successive projections algorithm coupled with linear discriminant analysis (SPA-LDA) classified correctly all samples, while the partial least squares coupled with SPA for interval selection (iSPA-PLS) quantified adequately the adulterant, attaining values of RMSEP of 11.64 mg kg−1, R2 of 0.96, RPD of 5.28, REP of 13.63% and LOD of 39.45 mg kg−1. Therefore, the proposed methodology provides a simple, fast, inexpensive, promising analytical tool for the screening of both the quality and safety of ketchup samples. As a consequence, it can help to protect the consumer's health.