Chlorine gas turns a damp starch-iodide paper from its original colour to a dark blue color. The reason behind this change is that chlorine gas reacts with the iodide ions in the paper, oxidizing them to iodine. This iodine then reacts with the starch in the paper, forming a dark blue complex known as starch-iodine complex. Therefore, the presence of chlorine gas can be detected by observing the change in color of the damp starch-iodide paper from its original color to a dark blue color.