Final answer:
The red dye in alcohol thermometers improves visibility, aiding in accurate temperature reading due to the significant change in the length of the column with temperature changes.
Step-by-step explanation:
The coloration in the alcohol used in thermometers serves a practical purpose. It improves visibility of the alcohol, making it easier to read the temperature. When the temperature of the thermometer increases, the dyed alcohol in the bulb expands more rapidly than the glass around it, forcing the liquid into the narrow tube, and this creates a significant change in the length of the alcohol column for a small temperature change. The red dye aids in distinguishing the alcohol level against the typically light-colored scale of the thermometer.