The best and quickest way ?

Nasturtiums are quick and easy from seed as are Californian poppies, both are very colourful. My faves are Empress of India and Red Chief.  Nasturtiums will lure away caterpillars from your brassicas, so planting some away at the side of the plot is a good idea 

I like calendula (pot marigold) as they are easy and quick from seed too and look fab in amongst the brassicas esp red cabbage.  The blousy African marigolds and tagetes are fine for the plot as they attract hoverflies which devour aphids, and they are cheap to buy as plants

If you need double quick then you could buy some plants and pop them in

Sweet peas are lovely on the plot, twining their way up your bean poles and these can be grown direct from seed wherever you want some vertical colour