Describing color—having edited and fact-checked umpteen dozen Horticulture articles and written up many "Plants We Love" for this website, I have determined it is the most difficult aspect of writing about plants.
I've come to believe color is somewhat subjective, at least when it comes to noting a very certain shade. Color also seems to tempt us to get spicy with words. It's so boring to just say "yellow." Let's get some adjectives in there...sunny, buttery, golden, the list goes on. Dark, pale, bright and soft don't seem to cut it. I try to encourage writers to keep it simple, but I don't like to step on toes, either. A lot of it boils down to personal writing style, plus common sense. If I think you will understand what the author means, and we have a photo to support the text, I will leave their words as is. Personally, I'm more apt to say "light pink."
One color in particular drives me nuts, because not only is it much sought-after by gardeners, it is also a real trickster. I'm talking about blue. So many times, we get a story in which one of our trusted and experienced writers describes a plant as "blue flowered." Then we start fielding photo options, and, "Gee, is it me or do the flowers look purple?" It reminds me of the story "The Emperor's New Clothes." Remember how everyone pretended they could see the emperor's clothes, though really he was naked as a jay bird? I want to see the flowers as blue; but I have to speak up and ask if they aren't.
Now, some of the above trouble with blue stems from technical matters of printing photos and/or viewing them on a computer screen. I have also read that flower colors can look different depending on where you garden, owing to altitude and latitude, which affect the intensity and angle of the sun and UV penetration, as well as humidity and temperature. But here I'll write from my own experience of looking with my own eyes at the plants growing in front of me.
In my own garden, I have two blue-flowered plants—um, except for when they are absolutely purple! It depends on the time of day. In the morning, when my garden is shaded, my 'Blue Pearl' Jacob's ladder (Polemonium 'Blue Pearl') and 'Blue Hill' salvia look their bluest. When the sun hits them (most of the day for the salvia; only an hour or two for the Jacob's ladder), they change. The Jacob's ladder looks very pale purple; 'Blue Hill' looks medium purple. It also depends on how far I'm standing from them as I look. (I also grow Salvia 'East Friesland', which came billed as blue but is as purple as the day is long.)
Above: 'Blue Hill' salvia is on the left. You can see how the shaded flowers look blue; the sunlit ones look purple. On the right is 'East Friesland', just plain purple all the time.
Above: 'Blue Pearl' Jacob's ladder (Polemonium 'Blue Pearl'). I took this photo around noon, when it is "feeling purple." (Has been in bloom since early April, by the way—I think owing to the very cool spring we've had in New England.)
Of course, other colors also look different depending on the time of day and intensity of light. I think blue just riles us the most. I wish I could offer a solution, such as to never claim a flower is blue again. But sometimes—not all the time, but sometimes—they are. Perhaps a footnote instead—borrowed from Bob Dylan: "We always did feel the same / We just saw it from a different point of view: / Tangled up in blue."