In thermal infrared bands such as Band 6 (Landsat 5 and 7) and Band 10 (Landsat 8), clouds typically appear bright (white or light gray) because they are colder than the Earth's surface. However, if clouds appear dark in your downloaded images, it is likely due to inverse thermal stretching, where higher DN values (colder temperatures) are mapped as dark, or due to software-specific auto-stretching that affects contrast.

Additionally, if you are working with raw DN values or unprocessed thermal data, you may need to apply radiance-to-temperature conversion and adjust the display settings to properly visualize the cloud temperature. Try modifying the grayscale stretch or inverting the contrast to verify the correct thermal representation.