A test for quantifying the contribution of colour and scale to the visual impact of buildings in the rural setting was developed and validated by means of a public opinion survey. The method was based on some psychological aspects of visual perception and on a simple image treatment performed by a well known and easily available computer program. Results were provided as numerical values, enabling the quantification of the impact for the two individual elements studied and for their potential interactions (aggregate impact, AI). Validation involved 44 images and 1,046 participants arranged into two groups, and the method proved to behave with consistency in predicting the AI of these elements, since the ratings assigned by the observers to the visual proposals examined displayed a high correlation with the predictions obtained from the application of the test. A general threshold for AI was drawn from the results, and such threshold could be transferred without major difficulties to authorities performing the planning and regulation of the rural landscape, since the test is simple and requires just a minimal training. Extending the application of the method to testing elements other than scale and colour and their AIs might assist the searching of a global indicator of impact that may satisfy the needs of landscape planning and regulation in the future better than the current tools.