What is the liquid in the column? If it's water, bad luck. First, you risk to break the glass below 0°C, and second, water has its minimum volume at 4°C, which means that around 4°C, the density change very little, so does the buoyancy, and you have no reading.https://www.engineeringtoolbox.com/water-temperature-specific-gravity-d_1179.html
The answer would take a different liquid that doesn't risk to freeze and does expand over the whole desired temperature range. But if you change the liquid in an existing thermometer:
- You must calibrate the plungers again.
- You risk to pollute the liquid.
From 17°C to 18°C, water's density changes by 0.15kg/m3
. Little pollutant detunes your thermometer. Any operation must be done cleanly, and a different liquid must be pure for reproducibility.
The coloured liquid in the plungers matters little, because gas tops it, so this liquid influences the plungers' volume little. Glass at the plungers expands little with temperature, less than water does, and it's stiff enough to impose a constant volume, so the enclosed liquid gives only a mass.
So you have some freedom to chose the liquid in the plungers, and a fluorescent one is possible, sure. Varied colours and combinations are easy.
Colour changing with the temperature: I've seen such an indicator on a fridge. It needs no other light than what the eye needs to see. I ignore the details.