Who needs E numbers and artificial colourings when you have the splendid, radiant palette of Mother Nature? Think of the vivid hot pink of a slender stalk of early season rhubarb, or the luscious magenta of a heavy, ripe raspberry; picture the coral, pearly inside of a freshly cut fig or the eye-popping green of a blanched broccoli stalk. These colours are something for the cook to get excited about; they make preparing a meal as much of a joy as eating it. It’s rather ironic that the slogan ‘taste the rainbow’ was adopted by Skittles to sell their sweets, whose artificial colours are a sorry simulacrum of the spectrum real food has to offer.

Latest on my list of ‘foods whose colours I would happily paint my house with’ are purple potatoes. I found these at a farmers’ market in Cambridge and snapped them up, their dark, purple-tinged skins giving little hint of the shocking, purple-blue colouring that appears once you slice them open. Where some vegetables lose their colour upon cooking (purple French beans were the tragic disappointment of summer 2014), purple potatoes remain gloriously vivid. They have a wonderful soft, fudgy texture, making them perfect for nearly all types of cooking, but I like them best in a salad, where their colour can take centre stage.

This is a very simple potato salad, but it’s a long way from the mayonnaise-laden, snow-white, gloopy concoctions you can buy in the supermarket. Chunks of white and purple potato are bound with crisp radishes in a dressing of lemon juice, olive oil and crème fraiche, peppered with fresh summer herbs to lift the earthiness of the vegetables. Chives are mandatory, of course, but I also add mint, and lemon verbena from the garden (see my recent post on unusual herbs). It has a wonderfully sharp, citrus scent that cuts through the creamy dressing beautifully. Its pale green foliage also contrasts nicely with the purple flesh of the potatoes, if we’re talking aesthetics, and when are we not?

A potato salad, purple or otherwise, demands a fat piece of oily fish as its crowning glory. I served this with smoked trout fillets, which I smoked myself because I’m a massive overachiever and also because I am in the enviable position of doing a PhD, which means that I can use ‘working from home’ as a blanket term to cover everything from ‘napping’ to ‘curing trout fillets in preparation for their stint in the smoker’. If you are less fortunate, simply purchase some good quality hot-smoked trout, salmon or mackerel fillets. Or, if you would like to emulate my charmed life, use Diana Henry's excellent recipe from her cookbook Salt Sugar Smoke. I also roasted some slivers of fennel to go alongside. Searing oven heat, a little olive oil and some seasoning transforms pale fennel bulbs into something truly luscious, their pale green hearts collapsingly tender, their edges crisp and burnished, that astringent aniseed tang mellowed by the cooking and replaced instead by a subtle, caramel sweetness. It works beautifully alongside the fish, and adds another texture to the plate.

Coral-gold trout; purple potatoes; the slivered pink edge of a radish. Caramelised fennel fronds; the jade-white heart of the bulb; the vital, verdant green of a tapered verbena leaf. This is a meal of contrasts, a harmonious medley of creamy root vegetables, sweet fennel, fresh lemon and the salty, savoury tang of smoked fish. It's a beautiful plate and a celebration of the end of summer. It's also a reminder of the sheer magic of natural ingredients, given minimal cooking and combined simply to offer a meal that is more than the sum of its parts. Finally, it is, of course, a reminder that sometimes our planet throws the unexpected our way...like purple potatoes.

Purple potato salad with lemon verbena, roast fennel and smoked trout (serves 4):

For the roast fennel:

3 fennel bulbs
Olive oil
Salt and pepper

For the potato salad:

5 tbsp olive or rapeseed oil
Juice of 2 lemons
200g half-fat crème fraiche
400g baby new potatoes
400g purple potatoes (or more new potatoes)
Salt and pepper
A bunch of radishes, sliced
A handful of chopped fresh mint
5 tbsp chopped chives
A few lemon verbena or lemon thyme leaves
4 hot-smoked trout (or salmon, or mackerel) fillets
Lemon wedges, to serve

First, roast the fennel. Pre-heat the oven to 180C. Cut each fennel bulb in half lengthways, then cut again lengthways into wedges about 2cm thick. Toss with around 2tbsp olive oil and some seasoning, then spread on a baking tray and roast for 30 minutes, turning occasionally, until tender to the point of a knife and caramelized.

Meanwhile, boil the potatoes in salted water for 15 minutes or until tender. Drain and leave to cool. Whisk together the oil, lemon juice, crème fraiche, salt and pepper, then toss with the radishes, mint leaves, chives and lemon thyme or lemon verbena leaves (if using lemon verbena, finely slice the leaves). When the potatoes are cool enough to handle, chop into bite-sized pieces and toss with the salad. Serve with the roast fennel and trout fillets, with lemon wedges on the side.