Pink Poppies, Lavatera, Mallow, Marvel of Peru 4 o'clocks, all but the poppies would benefit from a head start indoors right about now. Janet

If you could fashion something to climb on (either in the pot or some chickenwire fastened to the fence), you could grow Matucana Sweet Peas (grow to about 5' or so) along with a bottom filler of annual baby's breath.

Amaranthus 'Early Splendour' has gorgeous foliage colour and managed about 4 ft even in my UK garden last summer.
http://davesgarden.com/pf/go/53232/
An advantage of going for foliage is that you're not reliant solely on the plant flowering to create the effect you want. 'Illumination' is another variety with intense colour.

There are other Amaranthus varieties that lack the day-glo bracts of those two but still have purple foliage and also plumes of magenta/purple flowers. Have a look at the following URL (click on the pictures for full-page) and drool!:
http://www.raretrees.org/aman0219.html

I had thought of Amaranthus - Love lies bleeding - I had some growing in my yard last year and it was huge - kind of got away from me. The foliage idea crossed my mind and I was thinking of annual grasses also. I will look at the other amaranthus although I will likey have a tonne of love lies bleeding volunteers. I was also thinking of cosmos but wasn't sure if I should have all 3 pots the same or not

Thanks for the ideas

Fern

Yes, shorter sweet peas and also hollyhocks could be grown. They both give a cottage garden look.

Fern,

Several varieties of sunflowers would be tall enough. Some varieties would be too tall. But of course, sunflowers aren't "pink/fuchsia or blue or white".

Delphiniums come in that color range, as do lupines, but both have a somewhat limited blooming season. Morning glories also come in that color range, but they self-seed and become weeds. Because they like cooler weather, I think I would try the lupines and delphiniums. Foxgloves also have some pretty colors.

MM

I have lupins and miniature delphiniums in the ground in front of where I will put the containers - real delphiniums [tall spiky ones] seem to get eaten as they flower with an ugly worm - since I stopped planting them no more yucky worms. The sunflowers actually sound like a nice idea and would provide birds with something to eat - my neighbour had 8-9 ft ones last year on the other side of the fence. I have almost decided on a funky annual grass in one of the containers - neat seed heads - had thought of amaranthus in a second and maybe sunflowers in the third. Would that be too eclectic do you think?

Fern

We have grown morning glories for years in whisky barrels with a bamboo obelisk. A great show of colour. Although they do self-sow to a certain extent, I would not qualify them as "weeds" - you can easily pull out the ones you don't want early in the spring.