Psychophysical experiments were conducted to investigate the colour appearance changes between small colour patches and real room colours. The results clearly showed that colours appear lighter and more colourful for 100171 colours. A method based on CIECAM02 was developed to quantify the colour size effect. (C) 2010 Wiley Periodicals, Inc. Col Res Appl, 35, 284-293, 2010; Published online 15 January 2010 in Wiley Inter Science (www.interscience.wiley.com). DOI 10.1002/col.20575