Hmmm I am not too familiar with the effect different fertilizers have on bloom color. The most important thing to remember when your looking at those buds forming and the petals are green is the fact that chlorophyll is present in the petals, making them green. As the bud matures the chlorophyll leaves the petals and they eventually become whatever pigment they were originally. Think about leaves in the fall (or pictures of fall when you live in so cal) Leaves from different trees are originally red, yellow orange but chlorophyll pigment masks them up, so they appear green. When the chlorophyll is absorbed by the plant in the fall, the leaves show their true colors. The same thing happens in flowers. They are green until the chlorophyll is absorbed. I like to think of hibiscus colors as layers, you have lavender blooms that are red pigments and blue. Then you get interesting cvs like blue ballerina where it is only blue pigment being expressed, and not red. If there was red, then it would look totally different. I think that you wanting to work with brown cvs could produce some really interesting cvs. It is pretty hard to figure out what pigments are being produced to make the bloom appear brown. Take for example creme de la creme, which is a brown but has a purple overlay vs voodoo magic which is a brown with an orange/red overlay. Both are browns, but are probably brown because of a different concoction of pigments.
So going back to fertilizer and pigment, I think the most important thing is to give a fertilizer that is good for that type of plant. In the case of hibiscus they need that extra potassium to produce pigment. It makes sense that once you added that extra potassium to your fertilizer regime for HH had brighter blooms. With more potassium HH could make more pigment and thus have brighter blooms. Micro-nutrients needed for flower formation varies with species. In hydrangeas aluminum sulfate is needed for blue flowers because the protein that makes the pigment blue needs an aluminum atom to work, if not it is pink. If certain hibiscus need micro-nutrients to make specific color it would be interesting to experiment with it. Adding small amounts of different nutrients and observing the color intensity would be pretty interesting.
Im sure Charlie could tell us for sure the effect of different micronutrients have on flower color, but I think that HH colored up because you gave it the right ratio of nutrients it needed and was lacking before
