Coloring the liquid steps - question

[Bowlerhat]

I have a lava lamp with the red wax and clear liuid.

I know you should use food coloring,but actually, how many drops of food coloring  is "allowed" for a globe ? how many did you use ?

Will the dye change the lamp's density ?

When do you add the dye ? is it safe to add the food coloring when the lamp is running?or whould I wait until the wax cooled ?

Thanks,any help will be appreciated !

[Autumn]

http://oozinggoo.ning.com/page/liquid-swapping

[Bowlerhat]

Thank you very much ! :D