Most of the bright flowers in flower beds are half hardy annuals, so there is no problem finding a good selection.
If you are new to gardening, then a good garden centre or nursery is best choice as most of these will have good descriptive colour labels these days.
Seed catalogues are increasingly selling small plants (plugs) which will soon grow after being planted out in the garden. Don't buy mini- plants too early unless you have somewhere to keep them growing!
Many of the plants sold or classed as half hardy annuals are in fact either perennials or biennials that can be grown in the same way as half hardy annuals.
Petunias, antirrhinums, celosia, marigolds, godetia, salvias, zinnias, cosmos, nemesia, nicotiana, asters, alyssum, lobelia, ageratum violas, pansies.
Impatiens - busy lizzy, fibrous rooted begonias (semperflorens, tuberous begonias, stocks, Asters, nicotiana, alyssum, lobelia, pansies, violas.
Busy Lizzy, begonia, clarkia.