As a gigantic monolith of chakra, the Susano'o inevitably consumes an insane amount of chakra.
Susano'o comes in different colors for each user: Shisui's is green, Madara's is dark blue, Itachi's is yellow/red, Obito's is cyan blue, and Sasuke's/Indra's is purple.
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The different colors of the Susano'o show that each member of the Uchiha clan has a unique chakra color, as Ao recognizes the color of Shisui's chakra, noting that he'd never forget that color.
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It's unclear and unknown whether every individual in the ninja world has a unique chakra color or whether only members of the Uchiha clan have a unique chakra color, or if only Uchiha members with the Mangekyou Sharingan have unique chakra colors.
Since Ao recognizes the color of Danzo's Sharingan is unmistakably Shisui's, that implies that Shisui's green chakra color is extremely rare and quite possibly one of a kind, since Ao recognizes it as indisputably Shisui's, even though as a Byakugan user, Ao has seen many users with chakra, yet Shisui's chakra color stands out among all other chakra colors.
The standard chakra color for ninja is shown to be pale blue. Even when Ao looks at Shisui's chakra color, Danzo's chakra color is depicted as the standard blue color.
This could either mean that:
(1) The Uchiha clan is unique in that all clan members have unique chakra color.
(2) Only Uchiha members with the Mangekyou or Susano'o have a unique chakra color.
(3) Every individual in the ninja world, both Uchiha and non-Uchiha, have a unique chakra color, but Shisui's green chakra color is particularly rare.
The second might be more likely, since Ao probably saw multiple Uchiha yet Shisui's color still stood out, meaning that Shisui's color was unique among the Uchiha. Or maybe Ao simply hadn't encountered many Uchiha, since after all the Uchiha clan was used for domestic policing rather than on the front lines, and the Itachi novels explain that basically only the most talented and skilled Uchiha - those being Shisui and Itachi - were chosen to do ninja fieldwork, which reduces the chances that Ao would have encountered many Uchiha.
