Geum's are a first class plant for such a situation. Certainly they cover all your colour requirements plus their compact size makes them a delight in gravel/stoney settings.

Try Californian poppies - just sprinkle the seeds and they then seed themselves!

Assuming its in full sun, Helianthemums come in reds, oranges and yellows.

Rudbeckia and Coreopsis will do well in a sunny gravel garden and both have very long flowering periods.

Also Cosmos is a very easy to grow annual and flowers for months

And, as a balance, what about a grass such as Stipa arundinacea, which colours yellow and bronze in autumn. Phil J

There are several beautiful red flowered penstemmons. They're evergreen and can flower from June to November

There are several different crocosmia which have flowers varying from yellow to orange to red.

As its a gravelled area, I'd be wary of certain Crocosmias - Lucifer, for instance, multiplies at an alarming rate,and you will be removing the gravel to dig it out every year...

I'm surprised at you saying that about Lucifer Bamboo . The ones I've got stubbonly refuse to spread at more than a snails pace. You are certainly correct about most crocosmia though which need splitting at regular intervals.

Blimey, really, Anchorman? I've dug up a clump planted 2 years ago which had got 4 x 4 feet already this year and despatched corms from it to various people on here, and I am waiting for the weather to stop being so horribly wet to go and dig out a patch in a client's garden which must have hundreds of corms - they occupy a patch 5 feet by 5 feet, and I only thinned them out two years ago...

I'm amazed. I planted a single stem of lucifer in my garden about 4 years ago and this year it managed 2 stems!

Must be the conditions, surely - we're on good old London clay here.

I'm on a very light sandy loam