I think I may have the answers to the OP questions.
I have a lot of experience with blue/purple hibiscus and with hibiscus the colors change due to a few things.
One would be temperature/ light. Another would be fertilizer.
Here is a brief on Anthocyanins
Anthocyanins
The Blue, Purple, Pink, Red, Black Spectrum
Anthocyanins are best known as the red pigment in fall leaves. We all know that fall leaves change color in response to weather conditions, and this is the hallmark of anthocyanin pigments. Anthocyanins are plentiful in hibiscus, giving the flowers their many showy bands of color, as well as their tendency to bloom with different colors depending on many different variables.
Anthocyanins are Unstable
Anthocyanins are a much different kind of pigment. They are much less stable than carotenoids. Anthocyanins aren't safely enclosed in plant cells. They are created in the roots, dissolved in the water that makes up plant sap, and move all the way up the plant to where the flowers develop. The least little change in the environment or health of the plant affects the sap, which means it also affects the anthocyanins. This is why blues and purples tend to be so changeable in hibiscus flowers - they are produced by the notoriously unstable anthocyanin pigments.
Anthocyanins & Flavanols Burn Away
Colors Fade Almost Completely
A single anthocyanin pigment can be deep blue or deep red or anything in between, depending on many different variables. One variable we understand to some extent is ph. The levels of alkalinity and acidity in flowers change the color of anthocyanins from the blue ranges to the red ranges. Gardeners use this knowledge to change the colors of hydrangeas between pinks and blues by changing the soil ph. This means we should be able to predictably change the color of a hibiscus flower by changing the plant's ph then, right? Don't we wish! Many of us have tried this, but unfortunately, ph in hibiscus is dictated almost entirely by genes. Within a single flower, there will be pockets of pigment with different ph levels, creating different colors from the same anthocyanin pigment.
Anthocyanins Increase and Turn Red in Response to Cold
Red Anthocyanins Burn Away in Heat
Nature does do some things that alter ph and other anthocyanin characteristics of plants though. Anthocyanins function as a sort of anti-freeze chemical in plant sap. In response to dropping temperatures, plants produce more anthocyanins, and the anthocyanins become redder. For some reason, redder anthocyanins seem to have a more protective anti-freeze effect in plants. This happens in maple leaves, and it's the same mechanism in hibiscus flowers. Dry weather increases this darkening and reddening effect even more, as we see sometimes when mid-summer drought causes tree leaves to develop fall colors early. But it does take a certain amount of sunlight to create anthocyanins, so fall weather, with bright sunny days, less rain, and cold nights, is the perfect weather to maximize anothocyanins, to turn leaves their brightest reds and oranges, and to increase and darken the reds and pinks in hibiscus flowers.
Red Anthocyanins Increase in Cold
'High Voltage' is a good example of this effect. In the hottest summer, the flower is almost pure white, with just a hint of pink blush from anthocyanins. As fall weather cools the nights, anthocyanins increase, and the pink blush starts to deepen and spread. In the coldest weather, the whole flower fills with anthocyanins and turns almost completely pink. In mid-winter here in Southern California where hibiscus keep blooming until Christmas, the December flowers can all be so red and pink that at times we have difficulty identifying them.
Anthocyanins Degrade and Disappear with Heat
Some anthocyanins are very sensitive to heat. Enzymes in plant sap completely destroy several of the anthocyanins in hot weather and bright sunlight, and this creates the fading effect we see in so many hibiscus flowers in summer heat. These are the hibiscus that we recommend keeping in partial shade during the heat of the summer, like 'Sleeping Beauty.' Its anthocyanin pigments are beautiful in the cooler, cloudier times of year, showing a rainbow array of 5 different bands of color, and holding the color well. In summer heat though, the flowers have only 2-3 colors, and in bright sunlight, these colors fade very quickly. But a 'Sleeping Beauty' growing inside a house in a window that gets an hour or two of sun every day will bloom with a full array of colors that will last for 2-3 days. Inside a house, or in partial shade, the anthocyanins are protected from too much heat and sunlight.
Anthocyanins Increase Production and Show a Wider Range of Colors with Maximum Nutrition
Anthocyanin Pigments Develop Best with Optimum Nutrition
Anthocyanins are produced in sap by a reaction between sugar, or brix, and protein. Higher sugar or brix content in the sap and an ability to produce proteins plentifully are both requirements for a plant to produce maximum levels of anthocyanins. Although horticulturists keep looking for tricks to increase brix production in food plants, the most sensible and easiest way is to maximize your plants health in every possible way. Extra good soil, extra good nutrition, extra good care will, over time, increase both sugar and protein production, and you will see the difference in your flower colors.
Some hibiscus are very, very sensitive to this nutritional effect, and will even stop producing flowers all together if the sugar and protein levels in the sap gets too low. Others will produce flowers, but the colors will be pale, or some colors will be missing. The health of the plant really does matter when the colors depend on anthocyanin pigments!
Here are some pics of my Hibiscus in different forms. ( heat also increases spotting ( if plant has spotting)
Winter color ( this is same hibiscus I will be showing you) Heartbreak hotel is the name
PS I barely see this plant in winter color since I live in southern California and my plants are inside.
A tad warmer color
Usual heartbreak hotel color I guess you can say normal temp color ( it looks like this most of the time for me
)
Extreme heat color ( this bud was directly under a t5 bulb) This is the first time I have seen it look like this. It was an amazing surprise.
This is E ku'u morning dew in its cool colors ( again this is a rare sight for me. I believe it can get darker if it was outside)
On the left pic of it in a darker color mixed with some spotting that is forming
This is it at its best in warmer weather
Linear Mode
