I am fairly sure I have seen it written كتم (if the arabic font doesn't show up, I mean k rather than kh, neither a written and the softer t with two dots rather than the tall t) but I may have remembered wrongly, as I can't remember exactly where I have seen it written down and it must have been some time ago. It looks like a nice colour in the picture- much lighter than indigo but without the warm tones most of the other natural colouring materials give.
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