My love is true, but is that blue?

I'm pondering blue this morning as I walk the dogs along the bicycle path by the motorway. Not a lot to be seen in the sky, really. But, as someone used to say in my childhood, there's "just enough" in those sporadic inter-cloud patches to "make a sailor a pair of trousers."

When we tried to paint the sky at school, imagining ourselves budding Constables, we were told to notice "how the blue is deeper and darker in the higher parts, but much lighter and more transparent near the horizon."

It's true. I've noticed it ever since.

Since I am thinking about blue as I walk, blues accost me from several places. The smart, newish signs, strategically erected for the topographical edification of the cycling fraternity, pointing to "Pollok" or "Paisley" or "Maxwell Park," have strong blue backgrounds. Royal blue, I'd say.

On the motorway, a rescue truck is hitching onto the front of a broken-down bus whose sign ("FAST DIRECT SERVICE") is now a touch ironic. Both vehicles were variously painted in vivid commercial blues, cobalt that is almost turquoise, then ultramarine, then Prussian blue. But none of these sign-painters' blues, however bold, has the exciting effect on me that one minuscule flower in our front garden has.

I don't know the name of this plant, long in residence, and it is no longer sold at the garden center. It is clearly wild – a species plant unmodified by the vulgarizing tendencies of hybridizers. The man at the center suggested it was a kind of myosotis or forget-me-not. I suppose that is possible.

But the tiny star-shaped flower, and there are very few showing so far this year, is no bigger than one-third of a little fingernail, and looks more like a veronica or speedwell. It has a white pinpoint center, which enlivens the blue, as if it weren't already a startling, lucidly bright, indescribable blue on its own account.

Each year, this blue flower is a sign that spring is imminent. I wonder if we don't think of typical spring flower colors as yellows (aconites and daffodils), purples (crocuses), and whites (snowdrops).

But this is also a magnificent time for blues. And spring's blue flowers, low-growing and brilliant, cheerfully give the lie to the notion that blue is somehow a strait-laced or sad, cold hue.

The scillas and chionodoxa, the blue anemone blanda, the pulmonaria, and the hepatica: If these elated, enlightening blues don't make your heart go pita-pat, I want to know why. They are all "true" blues, and all fizz in the cool air like "little low heavens" as Gerard Manley Hopkins put it, talking about a clutch of thrush's eggs.

Robert Geneve, professor of horticulture at the University of Kentucky in Lexington, compiled a whole book ("A Book of Blue Flowers," Timber Press, 2000) about them. He also took the photographs. They're not bad, either.

Blue is a notoriously elusive color to capture just right, though fortunately the days are long gone when color photography couldn't tell its blues from its pinks half the time.

My Uncle Leslie, smitten with the glamour of taking color photos of flowers, was always apologizing as he clicked the next slide in the carousel. "This pink Meconopsis is actually a blue poppy. You'll have to take my word for it" – or words to that effect.

Professor Geneve's book jacket would have made Uncle Leslie drool. Here, like just-hatched butterfly wings, is the exact color of the blue Himalayan poppy. A miraculous blue, a blue of dreams.

The Ektachrome and Kodachrome of those early days, strangely, was not a million miles from a peculiar fact when it read "blue" as "pink." Of the comparatively few plants in the vast plant kingdom that naturally produce blue flowers, quite a number also produce pink flowers. Borage, for instance, and hydrangea. Forget-me-nots, the anemone blandas already mentioned, and the annual cornflowers.

Professor Geneve's book discusses and illustrates the most difficult characteristic of blue flowers (and it is in flowers, he points out, not in leaves or stems, that plants sing the blues). This difficulty is that no one quite agrees when a blue flower is indisputably blue. A great many "blues" are really violets or purples or greens. Working on his book, he asked his wife numerous times, "Is that flower blue?"

In the end, she made a reply he quotes: "If you want it to be, dear." Hmm, well, OK.

But there is a color chart in the book (the sort of circular chart that painters and color theorists fancy) that distinctly separates "blue" from "blue-green" and "blue-purple." Looking at flowers, however, the distinction is infinitely subtler and more tenuous.

I have nothing at all against "blue" flowers that are only halfway blue, or might be blue, and would be blue if they just blued up a notch or two. They all have their own charm.

But in the final analysis (and this is a stimulating game to play when confronted by a rattling good bed of delphinium hybrids, those gothic spires of flower-display), you do know, when you see it, which blue is neither a touch too violet nor fractionally too green, but is precisely, exactly, unfalteringly and piercingly blue.

And this blue is a rarity by no means to be sneezed at.