Brilliant Blue is often used to trace water movement in soils. Its concentration in soil samples is usually determined by extraction - a laborious procedure with varying accuracy. In this feasibility study, we show that Brilliant Blue can be estimated directly by visible (vis) diffuse reflectance spectroscopy (DRS). We build a PLSR (partial least squares regression) model for the concentration range of 0.1 to 15 mg Brilliant Blue per g soil with an RMSE (root mean square error) of 1.35 mg g^-1 and an R_adj² of 0.88. However, as the method is based on visible spectra, prediction accuracy can be altered by variations in soil colour between calibration and prediction data sets. Therefore, we recommend a site specific calibration based on typical soil colours and an expected range of Brilliant Blue concentrations.