The Modified Uniform Oil Colour Scale (MUOCS) is proposed for the colour specification of virgin olive oils. MUOCS has 60 standard colours, the same number as the two previous scales available for the same purpose: Uniform Oil Colour Scale (UOCS) and Bromthymol Blue (BTB). A remarkable improvement in accuracy can be achieved from MUOCS standards: For a broad dataset of 1700 virgin olive oils produced in Spain during four different harvests, the average colour differences to the closest standards were 2.86, 3.99, and 8.17 CIELAB units using MUOCS, UOCS and BTB, respectively. A low-cost (<60 Euros) portable electronic device is proposed for the colour specification of virgin olive oils based on MUOCS and BTB standards. This device can operate with USB connection to a computer or AAA batteries, and is based on fast (<0.25 s) transmittance measurements of the virgin-olive-oil sample placed in a 5-mm pathlength quartz cell. The device may avoid the use of expensive laboratory instrumentation for colour measurement and can be easily employed by non-technician users. For a reduced set of commercial virgin olive oils with random colours, the MUOCS classifications provided by our device agreed with those given by conventional spectrophotometric measurements in 92% of the cases. (c) 2012 Elsevier Ltd. All rights reserved.