This tool in mainly a placeholder, so anotate is not selected by default.
For some reason this tool is not selected, unless it is added to PREVIEW and SEQUENCER_PREVIEW.
Is there any rush to commit this as a place-holder?
Check the image editors eye-dropper, that can be used as a template for the eye-dropper here too, internally work the same way & share code.
I bit misremembered your comment and started looking at eyedropper tool of greasepencil, since image doesn't have eyedropper tool...
Now I am not sure if you wanted to make both sample tools (vse and image) be part of interface_eyedropper_color.c or if I should make new template like interface_sample_color.c
Only problem integrating sample functionality with interface_eyedropper_color.c was:
I don't think you need to touch interface_*.c files. Check SEQUENCER_OT_sample and IMAGE_OT_sample, some parts of the image operator may need to be replicated in the sequencer eyedropper for it to be useful as a tool.