So I ran across this and thought that it may help a few people out.
Highly dependent on Nitrate and PO4 levels. Of course all SPS colors are highly dependent on lack of N and P so I wanted to start with probably the easiest color to get, yellow. Yellows are sort of you baseline; yellows will tell you a lot about what is going on in your tank, what is needed and what is overdosed. Nitrate and/or PO4 reduction is most important, either through technical means such as nitrate/phosphate reducers or biologically through DSB, Carbon dosing and/or water changes and fuges. Basically, if you want to do SPS, I would suggest starting with an acropora that is yellow. If you can get it to say yellow for several months, you should be ready for something else.
Greens would be the next easiest color to tweak. Most green coloration can be achieved through the addition of an Iron Concentrate (Kents is what I use, however Iron is Iron). You must be very careful with Iron because it is also an Algae accelerator; this is why it is so important for you to get your yellows colors first (your N and P will be lowered).
Additionally, I use my yellows as indicators for my greens and blues. You'll notice a deficiancy if your greens are brown color or they are paling in color. I start off by dosing Iron at about 1 drop per 50 usg twice a week and take note of what happens, color changes, Algae growth, until my yellow acroporas display a green shimmer (it won't be a solid green but a shimmer of a green/yellow).
Please note, a sign of overdosing is a darkening of tissue, when this happens you have added too much iron or too much iron is being added. Another sign of overdosing is Algae growth, stop immediately and possibly do a water change if necessary. Like everything else reef, go slowly.
Blues and some purples
This is mainly for blues but I have found is can also have an effect on purples. The supplement for this is Potassium Iodide Concentrate or Lugol's solution, ESV Potassium Iodide Concentrate will also work; don't just get something that says Potassium because that is a little different. Dosing should be done when blue colors become less intense. Again, using yellow corals as indicators, stop dosing when yellow corals display a green shimmer.
Reds/Pinks and some Purples
Primarily for coloring reds and pinks in Montiporas, Pocilloporas, Birdsnest, other Stys and Seriatoporas. The supplement is Potassium (not potassium iodide). If you are using a high potassium salt mix such as Oceanic, Tropical Marine Pro and you are doing regular water changes, you are more than likely not going to need to supplement this much.
For dosing you can use your monitporas, especially caps as indicators. Supplementing is required when Montiporas display slower growth and appear washed out to grey appearance. Indicators on Stys and Pocs are when they look like they have been exposed to air. Polyps are completely withdrawn and colors are light. Other indicators of potassium deficiency is when the pinks turn into a light brown and when acroporas loose their color and get lighter and pale. A major potassium deficiency is seen when tissue is lost, mostly starting from the base opposed to spotting (patchy look). And overdose can lead to tip burning so don't mistake tip burn for new growth. Tips burns will be white with no polyps.
Probably one of the hardest coloration of all acroporas from my experience since it is a combination of several variables.
First and foremost is water clarity, which means Carbon and/or filter socks. I have also had good result from biological filters such as using cryptic zones, which produce seasquirts, sponges and other filter feeding animals. Zeo Sponge Power, which can be used in any system, feeds sponges. Sponges are great because they can filter a mass amount of water for better water clarity.
From what I have noted, increased water clarify will first effect SPS tips but not the complete base. I have seen nana and valida with really nice purple tips but brown/tan/white bases. I have seen the same nana and valida in another's tanks, which met all other parameters with a full purple from base to tip.
Second being lighting. From my observations of my own tank and others, purples seem to love 420-440nm range light spectrum, those found in actinics and 20K halides. Some of the best purples I have seen are in tanks that have 440nm blue actinics (ATI Blue+, Giessman Actinic) or 20K Halides (Radium, XM 20K).
Third, supplements such as Iodide and Potassium (see blues and Reds/Pinks). Again, make sure your greens are green and yellows are yellow. Your blue should be bright with depth. Iodide will also help if you have tip burn.
These are just my observations through testing and I am sure in the future other factors will be seen and added. Please feel free to comment with your own observations, data is very important to moving forward.