Four-O-Clocks are a lovely mass planting around a tree or against some
other feature. They bring butterflys and humming birds and also have a
delightful perfume which you can notice in a fair size planting. All
plants are weeds if they grow where you do not want them but these
plants can be transplanted easily and you can also control them by
picking the seeds as you suggested. Plants with pigmented flowers have
the same pigment in the whole plant and you can be sure the ones with
purple leaves will have darker flowers although, in the broken colors
they still may be veri-colored. If you grow the saved seed you may
need to nick or scarrify it and soak it before planting. Have fun and
be assured, most people who see your four-o-clocks will love them.