sungit, you can use [color = white] for this effect:
can you read this? (ideal for light gray background)
or [color = silver] for this effect:
how about this one? (ideal for dark gray background)
Though these aren't totally invisible, the text are still partially hidden and their color codes easier to remember since you're using actual color names.
If you still prefer the 100% invisible effect, just remember that color code is either six E's or six D's.