Hey dakims and cyberajnabee
I have attached the acid-base dependent equilibria of malachite green in a jpeg file. Upon treating malachite green with basic water the carbinol is formed and this is colorless, since in this form, none of the benzene rings are conjugated.
The reason why the color is diminishing upon treatment with acid is, that one of the amines is protonated, giving the acid salt. The acid salt has less conjugation than malachite green since the lone-pair on the nitrogen is now used in a bond to hydrogen and it is therefore also less colored.
Rico