The grey uniforms were Wolseley's idea. He was big on more practical uniforms for the tropics and his troops in the Ashanti war had worn an experimental grey uniform, designed to be less visible and more comfortable, and it had been considered a success.
Consequently he had ordered grey uniforms for the Egypt expedition of 1882, but they failed to arrive in time and so were in storage in Egypt when the Sudan crisis erupted. They were issued to the troops sent there from the UK, as unlike the troops from India, they had no khaki uniforms.