Can't even change the pallette in the dinky terminal colour picker.
Can change it in toolbar colour picker (the one the OP is talking about) but it doesn't persist. Note on LM this is the good version of the picker by default, not the same dinky one as the terminal picker.
Looks like it may depend on the default system/theme/GTK version (GTK 3.24.20, GLib 2.64.3) here.