Why take all the trouble to dye silk when silkworms can be genetically modified to spin any colour of the rainbow?That's the goal of Japanese scientists who have genetically engineered silkworms to produce a specific colour, according to a study published this week in the Proceedings of the National Academy of Sciences Lead author Takashi Sakudoh, of the University of Tokyo , says understanding the pigment transport system of silkworms could "pave the way for genetic manipulation of the colour and pigment content of silk".For starters, the researchers have produced silkworms that make yellow silk. But they say that in the future, the worms could be manipulated to produce flesh-coloured or reddish silk.In nature, silkworm cocoon colours vary from white, yellow, straw, salmon, pink to green.The colours in the silk are from natural pigments absorbed when the silkworms eat mulberry leaves.And the genetics behind this ability to extract natural pigments is crucial, the scientists say.For instance, a gene known as 'yellow blood' or Y gene enables silkworms to extract carotenoids, yellow-coloured compounds, from mulberry leaves.And worms that have a mutated Y gene, where a segment of DNA is deleted, cannot do this so produce white silk.Insects with this mutation produce a non-functional form of the carotenoid-binding protein (CBP), known to aid pigment uptake.Using genetic engineering techniques, the researchers introduced pristine Y genes into the mutant insects.The engineered worms produced working CBP and yellow-coloured cocoons.The yellow colour became more vivid after rounds of crossbreeding, the researchers say.