Q:

How can I learn the name of a colour to use in a description?

I am trying to describe this character:

Source
Her attire is pretty easy to describe, Purple and Gold (while it looks yellow she addresses herself with majestic plural, so gold would be more fitting) clothing with a black coat. 
However, her hair is a bit of a problem. I want to describe its colour almost exactly as it's seen here and while initially one could say it's like a silver colour there seems to be a bit more of a cream color.
This hasn't been the first time I've gone to describe a colour, however other times I've known the base colour (Red/Blue) and from there looked up on Wikipedia to looks at its shades. However, that won't help here when I can't be sure what base colour her hair colour would be in.
So I am wondering, is there a place online where I could submit a hexadecimal value and get results of similar colors? (eg. if I submitted #FF91A4 I could get Pink and Salmon) That way I can use that in the future. (Since obviously there isn't an SE Site where IDing colours in an image would be on topic.)
NOTE: as you can probably guess, the character is "Anime Style". As such natural hair colours don't follow real life norms (some characters can be born with blue, green and pink hair).

A:

For color-to-name converter, a quick Google search gives me this link:
http://chir.ag/projects/name-that-color/#C0C0C0
In which you can just pick a color from the color wheel to see its name.

Perhaps the color you want is "Mercury"

But, as Phillipp said, you might better explain the color in words more frequently used, instead of using some rather pedantic terms. You can say:

She has a gray hair

or if you want to invoke more imagination from your readers, you can try something like:

Her hair color reminds me of my highschool year, when I accidentally broke a thermometer during one science experiment, causing the liquid inside to spill over all the places, covering the floor with the shiny silver color of mercury.

A:

I love the apps linked in the other answers, but I see a basic problem with this whole question.
Let's try an experiment.

Choose three color names that for you describe a very specific color.
Now give these names to a few people and ask them to select all the colors that would fit that color term from this color chart: http://www.pantone-colours.com/
How wide is the range of colors for each term? Does it include the color you thought of?

I'm sure you can imagine the outcome of this experiment: Everyone has a different idea about what a color term signifies. Because in fact individuals perceive color differently. Friendly arguments are fought about where exacly blue and green begin.
So trying to exactly name a color with words is futile. Everyone will imagine what you describe differently, and there is nothing you can do about that.

