For an objective measure we expect/demand a measurement standard. There are standards that would say one way or the other is the only way, while there are other

For an objective measure we expect/demand a measurement standard. There are standards that would say one way or the other is the only way, while there are other more relaxed standards that say either is acceptable. If you know the standard measure you can objectively say if "color" is correct or not. For example, if we accept Merriam-Webster as our standard measure then in each case both spellings of color/colour and indexes/indices are acceptable. One might be preferred over the other but both are objectively correct. We would then conclude that an author writing for a US audience (per the standard measure of Merriam-Webster) would not have erred in using colour (or color).