without going into teh ethical question, I know what colourful things I like in my garden, maybe that will help...
lavender is always good and hydrangeas
There are some flowers you can sow now that will come up next year, (check these as not 100% sure) like hollyhocks (OK cheated there I didn't know that till just now), poppies, californian poppies, lunaria (think its real name is Honesty).
And of course all the bulb flowers can go in soon!! Tulips, daffs, hyacinths.
All the spring flowers of course will have to wait but you have all the brightly colored daisies, zinnias, nasturtiums (they give great ground cover) etc.
Why not try a gardening calender to see what they recommend sowing at this period? for example from the RHS site;
"Collect and store seed of hardy annuals and perennials for sowing later in the autumn. Good plants to try include Calendula, Nigella, Cerinthe, Papaver, Aquilegia and hardy Geranium. Allow some plants to self-seed to give an element of surprise to the garden.
Towards the end of August sow hardy annuals directly into borders. They will overwinter and flower next summer. These include:
Hardy annuals requiring no protection in the south
Calendula officinalis
Centaurea cyanus
Limnanthes douglasii
Linum grandiflorum
Hordeum jubatum
Nigella damascena
Lunaria annua
Papaver commutatum, P. rhoes, P. somniferum
Briza
Consolida
Buy or order spring-flowering bulbs. Some bulbs can be planted now, such as Colchicum, daffodils and Madonna lilies (L. candidum).
There are lots of gardening calender organised by month on the Net.