Since the previous commit, the `color_field` needs to be explicitly stated
in the options for the tags to be colored.
The previous behaviour (always read the field `color`) was leading to server
warnings if the field was not present on the comodel.
Thìs commit set the `color_field´ for every many2many_tags on fields that have
a `color` field on the comodel.
Martin Geubelle
a29f7f671b