It's a phrase I've seen repeatedly come up in Japanese media. Namely I've been focusing on how the term is applied to entertainment media - fictional characters, idol members, etc - but in a broader sense, while googling イメージカラー, I've also seen it used when talking about the colors a corporation uses to represent itself, or in fashion advice for real people.
Because I can't actually read Japanese, and am relying on google translating interesting-looking links, I don't know how to begin researching the full extent of what concepts the term encapsulates or where it "originates" from. Japanese Wikipedia mentions that "member colors", image colors for individuals in idol groups, are directly inspired by the character colors used in Super Sentai (the source material for Power Rangers) and similar fictional entertainment.
A Japanese internet quiz I found opened up with:
An "image color" is a color that matches a person's image.
For example, have you ever had the impression that passionate people are red and intelligent people are blue ?
This time, we will diagnose your image color with 10 questions
about your personality, character, etc.
And another website:
Colors, whether it be in fashion, interior design, or the colors of a city, not only create the impression of a situation, but also affect people's psychological state. This includes initiatives such as corporate colors that take advantage of the psychological effects of color. The same is true for individuals, and by skillfully incorporating image colors, it is possible to express the person you want to be. So, what kind of psychological effects can different colors have? In this article, we will introduce basic knowledge about colors and the different impressions each color gives.
The benefits of wearing your image color
Your image color has the effect of conveying the personality and impression you want to express to others in an easy-to-understand way. Here, we will explain the benefits of wearing your image color.
What is image color?
The color that expresses a person's character or personality is called an image color. Celebrities, idols, anime characters, etc. have image colors set in order to highlight their individuality and make them easier to remember.
While these examples focus on the typical symbolic associations between color and personality (such as red = passionate and blue = calm), fictional works may intentionally subvert the associations between an image color and an expected personality or archetype from time to time, or apply them for reasons having more to do with some specific fictional lore than typical IRL symbolism.
For what it's worth, the Japanese Pixiv Wiki page for characters in Homestuck refers to the human character's text/shirt colors as their イメージカラー. That's actually part of what inspired my quest to understand the concept - western media can have "image colors" too, we just don't have a discrete term for "this specific color symbolizes this specific character within the media itself, within its official merchandise, et cetera." It feels like a useful term to import, for the discussion of this specific flavor of color symbolism and its relationship to character design.
