That happens towards autumn usually, so either the weather conditions are very cool, or they've not had enough water.

Mine are also starting to go this may and yes agree with Bamboo as we certainly have not had much warm weather.

Mind you the red leaves are looking stunning.

Some Fuchsia varieties have red leaves naturally.

This also happens if plants are put out into the cold from a sheltered greenhouse without being hardened off first. They should regrow green leaves eventually.