I would argue that while you can use colour to help your documents should be
* Perfectly readable when monochromatic
* Fully understandable without the colour
I would be concerned if you need to add colour for your data to be understood, and if you add it then it should be done with the understanding that not all of your readers will be able to see the same differences as you can (and you might be causing issues by altering things like contrast).
My only other main comment on this is similar, I want the communicated information to not rely on anything visual in tables. Bolding some results to indicate that they're significant is fine, if that information is available in another way, as I hate converting a format and losing information.
color-wise, one of the strongest reasons to not use color is the exact situation you mentioned; in some countries (namely, china), green values are negative and red are positive.
If all units in a column are the same, mention that unit in the column header. Otherwise, you need to mention it everywhere. Only putting units in the first row suggests that the first row is different. It is basically lying in all the other rows, presuming the reader catches the error and understands what is meant.
I also think disagree slightly on color. Restricted use of color makes sense For example red for negative values, or green for highlighted values that you want to stand out.
Heck, if you have very geometric 2-D data, perhaps there is even a point to be made to color the background according to the value so you can 'see' the structure. This is pretty niche though.