On days with thinner cloud it is possible to get a little directional sunlight coming through, which creates stronger shadows which can still be soft as long as there is cloud in front of the sun. Bright overcast is an almost ideal compromise between the strong contrast of sunshine and the relative dullness of heavy cloud.
On days with thinner cloud cover the sky can have a lot of texture, whereas on days with heavy cloud it tends to look a solid white or grey. Varying cloud thickness or small gaps between clouds can also help to introduce colour into the sky, with blue skylight and yellow sunlight reflecting onto the surface of the clouds. Colours in the sky can vary enormously when cloud is thinner, and the sky can often be very striking with thin or broken cloud. Another factor influencing cloud colour is that distant clouds can appear yellow or even orange because of light scattering, even in the middle of the day.