When the artists Jake and Dinos Chapman bought a series of paintings by Adolf Hitler for £115,000, many questioned the morality of paying for works produced by one of history's most brutal dictators.

Yesterday, the brothers unveiled 13 of the watercolours, on which they had added psychedelic rainbows, stars and love hearts, and placed them back on the market for £685,000.

The Chapmans denied that the paintings, which are selling as a single work, sought to "redeem" Hitler, and classed the original watercolours as "bland" and lacking in talent. "The idea of redeeming Hitler is bad, the idea of redeeming his work is a staggering work of genius," said Jake.

He hoped the defacement of Hitler's work, which includes landscapes, vistas of Roman ruins and still life, which the dictator painted when he was young, would have him "spinning". The changes they had added meant it was no longer Hitler's work, he added.

"If hell exists and Hitler exists in it, he would be spinning if he saw these. It's not his work any more. It's our work," he said.

Following concerns that the work could be bought by Nazi sympathisers, White Cube Gallery in London, where it is showing, has stated that it will be extremely careful about who it sells to.