In studies of building stone soiling and decay, as well as in other areas of building research, there is a need for an inexpensive and reliable method of assessing colour across surface areas. An integrated digital photography and image processing (IDIP) method has recently been developed and is presented here. This method enables cheap and reproducible measurements of lightness and chroma, using the L*a*b scheme, across a range of sample sizes. Of notable advantage is its allowance for areal rather than point sampling of surfaces. The IDIP method has been tested on limestone sensors which have been exposed between 1 and 5 years in the city centre of Oxford, England as part of a bigger project and for which spectrophotometric data are also available. Mean values of L*a*b derived from digital images using Adobe Photoshop were compared with similar data collected using an X-Rite SP68 sphere spectrophotometer. The IDIP method produces less variable data than the spectrophotometer, and there is good correlation between the two methods for lightness values. Chroma measurements using IDIP are less reliable than those collected using spectrophotometry. Both methods indicate that soiling of the exposure tablets increases dramatically over 5 years, with notable increase after 4 years.