Remove Duplicated 'gray' colour from ColorType

[lb803]

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This is a very small PR to remove a duplicate "gray" entry from the ColorType constant.

While browsing the code, I noticed a duplicate entry in the definition of ColorType and decided to open a PR to rectify it.

I considered adding a unit test to ensure that duplicates won't occur in the future, but this seems challenging due to the the Literal class in python, which parses input parameters using a _deduplicate() function that removes duplicate values. My attempts to check for duplicates in ColorType always returned that there were none, so I am holding off on this for now. Please let me know if you have any opinions or suggestions regarding this.

Thank you.

[picklelo]

nice catch!