Exploring opacity and transparency in Winsor & Newton Professional Acrylic, highlighting how important it is to consider this quality as much as colour. Two reds are compared, Cadmium Red Deep (opaque) and Quinacridone Magenta (transparent), by pulling them down a white board and across a solid black line to show clearly the distinction in coverage between opaque and transparent colours. The opaque colour obliterates the black whereas the transparent colour allows it to show through. This demonstration is repeated with Cobalt Blue (opaque) and French Ultramarine Blue (transparent), and then two opaque colours which are traditionally used for underpainting are also shown: Red Iron Oxide and Chromium Oxide Green. Since opaque colours provide good coverage, they are best for covering large areas and underpainting, whilst transparent colours are best for glazing.