Aberrant colouration in the European Tree Frog in the Netherlands Although bright green is the characteristic colour of the European Tree frog (Hyla arborea), other colours may occur. We report on brown, grey and blue individuals, as well as green ones with spots of these colours or a yellow dorsal stripe. Brown seems to be common in cold weather. Recently, a new colour variant was found in a frog that was seen in two successive years: greyish white with little green blotches and tiny black flecks. Readers are asked to report any other observations of aberrant colours.