I would do it when it is cold.
And i would first put the color in a glass of distilled water (or the water of the lamp).
With my food coloring, the color drop is going down to the bottom and is slowly coloring the water.
We do not recommend coloring the water. It is very hard to find companies that make 100% water based dye. If you do find a fully water based dye you also run into the chance that the may have used regular water with their dye. All the impurities that are in "tap" water is what clouds up lamps.
I am not saying it is impossible. It is just risky. In my opinion there is nothing is quite like a crystal clear lamp. It is the "magma" that is so mesmerizing. Not the murky water.
Marcel has some good suggestions on changing the water color. Diluting it with more distilled water is definitely the way to go.
Hope you all had a great New Year!
Again, Neither Magma Tower or I recommend you change the water color.
In two original Mathmos Astros, one (supposedly still original) 1990ies Crestworth and also in Lamps I restored myself, I have, successfully, used the ink you get for the DIY-Refilling of inkjet printer cartridges to color the 'water'.
It takes only a few drops for a good saturation and you can mix your desired color following basic rules applying to mixing Cyan, Magenta and Yellow. The coloring also turns out to be stable and fades much much slower.
Of course it is always an experiment at your own risk and, by all means, you should always test it in a bottle of water first, as to find out how many drops of ink you need for the saturation you want, because it is easy to overdo the coloring!
You can always add yet another drop, but you can not take out one too many!
in late August, I used a small screwdriver to puncture the cap, a McCormicks neon food dye in a syringe & added the colors one at a time. I thought it turned out interesting & my color hasn't faded... till the summer sun comes this year ;D