As dark colour stays permanently on the white cloth.
Explanation: This line has been taken from the poem 'सुभाषितमधुबन्दवः' In this shloka, it is said that dark colour stays permanently on the white cloth, means it can not be removed. In the similar manner, a human being should speak where his words carry importance and speech is fulfilled.
This line indicates that a wise person should present his opinion or views in front of considerate and learned people but he should remain silent in front of fools. What is the use of speaking in front of fools? As a dark spot on a white cloth stays permanently and can not be removed, in the same manner, a dark spot on a human being's character also becomes permanent. Such a person is condemned everywhere.
Here, poet advises that a person should speak only where his words carry importance. One should not speak anything anywhere. Sometimes depending upon circumstances, one should remain silent to maintain his self-respect. Here, the human being is advised what to speak, when to speak and where to speak.