api: apply OIIO_NODISCARD to color.h (#5196)

Adds `OIIO_NODISCARD` and `OIIO_NODISCARD_ERROR` to `color.h`.
Deprecated functions are left unannotated.

Part of #4781 

Assisted-by: Claude / Opus 4.7

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Signed-off-by: Luna Kim <177369799+luna-y-kim@users.noreply.github.com>

Author: luna-y-kim