We have recently added a new feature for specific cloud tint colour (on the trueSKY Sequence Actor), however to use this you will need to be using the new Variable grid integration scheme – which is a work in progress and can produce obvious errors.

Alternatively you can adjust the Light Wavelength (in the Sky Layer options) which will adjust the colour of the Sun, that will adjust the colour of the overall scene – however this is not a direct colour change for the clouds.