As far as I know, the colors of the popup´s diagrams cannot be specified.
However, it seems that the system´s default colors are assigned in a systematic way – I suppose as a function of order of the columns of the attribute table that are used for the diagrams. Hence, knowing this order would allow to adapt the columns´ order in a way, that the colors might fit the user’s needs as well as possible.
Does anyone know the order of colors used by AGOL.
thank you and best regards,