Main post on Coloratura.
When I was first creating the mechanics for the synesthesia of colors and color-influencing, I decided that each color (or single wavelength) was to directly represent a single emotion. Then I had the devious though that to get players into an otherworldly mood, I should include colors that were outside the human-visible spectrum. So I created a color template that included blissful white, crazy black, apathetic grey, scared infrared, furious red, revolted orange, curious yellow, joyful green, caring blue, protective violet, and commanding ultraviolet. I made it halfway through writing Act I before realizing that I had left out indigo. This was actually a blessing, because I was frustrated at my inability to include just one more distinctive color/emotion combination: Trusting.
I had been having trouble wedging in "Trusting" because of writing and lore-realism concerns: I wanted to maintain that one-wavelength-one-emotion formula, but I was really out of colors. There are plenty of other colors on either side of the light spectrum past infrared and ultraviolet, but their names don't invoke the mental image of a color: "X-rays," "gamma rays," "radar rays," "FM," "TV," or so on. They are technically colors, but it was too ridiculous of a stretch to ask players to imagine them as such. It would be a hard sell to convey what exactly a "gamma-colored Captain" meant. Plus, their names were way too human-centric for my liking.
I considered including the color brown, but the "color" brown is actually some combination of red and green. If single-wavelength colors were notes, red might be a C note, and green might be an A note, but brown would be a chord. This means that I couldn't give The Aqueosity the ability to discern or even conceptualize "brown" - multiple colors would just represent multiple emotions.
I also toyed with including the "impossible color combinations" of blue-yellow and red-green, but because they were "chords" like brown above, they would have also violated the rule of one-wavelength-per-emotion, and, more importantly, it's a too obscure aspect of human neurobiology.
Luckily, as I already mentioned, I had forgotten indigo. It was a fortunate omission that gave me the final emotion-color that I needed.
I had one other major problem with colors: how to convey their meanings to the player. Technically, the Aqueosity just knows colors and what they mean. But players didn't. So it was important for me to convey right away just exactly how all the colors worked. Especially with so many colors (11 in total).
First off, I always paired a color description with an emotional hint to help reinforce the color's meaning. I tried to vary colors and represent them as often as I could. In the berthing, I gave the players a colorless Drifter that they could manipulate and color as they saw fit. I included this mechanic so that if some players wanted to make themselves a color key from experimentation, they could do so.
I was initially hesitant to give a list of colors with the command >COLORS, but it was of course absolutely necessary. The PC knows what each color means. It's infused with that knowledge. So that list needed to be accessible to the players with a simple command. I made sure to give that command to players at every help opportunity: in the >About menu, in the >Hint/walkthrough commands, and in error messages. It was important to make this as accessible as possible.
Another half of the coloring mechanic was dealing with correct syntax for the coloring command. It needed to be ">Color (target) (intended-color)" but not everyone immediately realized that this was how they needed to phrase things. The game had to intelligently parse out commands like like >Color Captain, or >Captain blue, or >Color blue, that the player was trying to >Color Captain blue. I had to agonize over all the different kinds of ways that players could attempt to color a thing, but phrase it poorly, and then gave an instructive error message instead.
All-in-all, I feel that the whole coloring mechanic was really a lot of fun and turned out very well. It added a lot of flavor and interesting narrative to a game that ended up being actually fairly linear. It was not necessarily the cheapest mechanic to implement, but its gains in enjoyment really made it worth it.