What is INFRA+? What do the colors mean? And if they are cloud tops: Why are they only shown for a small portion of the actual cloud cover?
On Satellite layer, INFRA+ is the brightness temperature measured from cloud tops in Kelvin (K). On desktop it is possible to change it to °C or °F.
On the mobile app this temperature is only expressed in °C.
This temperature gives an idea of the cloud top altitude and so of the risk of severe thunderstorms. High clouds tops gives thunderstorm and similar strong convective weather.
To complete what @idefix37 said, with the infrared, the colder the tp the higher the cloud is so potentially it could be a cumulonimbus type of cloud associated with thunderstorms but cirrus is also high and cold cloud and not associated at all with thunderstorms. As always in meteo, before taking a conclusion you should check other paramters, for instance check the visible channel as well to see if the cloud is thick or not. If not, what you are seeing with the infrared is probably a cirrus