Hi [USER=143743]@CatalinM[/USER]
There are many variables in your project and not clear exactly what you want to control. e.g. you want to control 8 global colors yet do you need to control the positioning of those colors (a bit more complex). So I am going to make some simplifying assumptions. That being that you want the pattern you see and that you want independently control colors (e.g. Hue, Sat, Brightness) to 8 different values. I am also going to assume that you do not need the very subtle background pattern (within each circle) to be controlled separately and you want those colors to track as you have set the global colors. If you need more control then that please clarify as the solution for you to consider below does not cover those other types of variation.
The first step is that I want to be able to isolate the different colors. There are various approaches that could be used (e.g. as [USER=14137]@Peano[/USER] has descirbed) and I show another.
First i turned the Layer into a Smart Object so adjustments can be edited later if desired. Second I am going to turn the colors into their equivalent B&W luminosity values. I am doing this by applying a Layer Style with the Color Overly checked and the parameter settings is that the color can be any pure grayscale value yet the blend must be color. This does the desired conversion.
Here is the image right before applying that color Style
[ATTACH=full]140487[/ATTACH]
Here is the resulting image after applyng the Layer Style. I also show the Layer stack and the histogram. Note that the Luminosity range does not cover the entire area from black to white (to be fixed in next step)
[ATTACH=full]140488[/ATTACH]
To spread that out I apply a Curves adjustment Layer and change the black and white points and as well to even out the histogram, I applied a brightening at the high end as shown below.
[ATTACH=full]140489[/ATTACH]
Now to limit this to just 8 levels, I apply a Posterization Adjustment Layer with the value set to 8:
[ATTACH=full]140490[/ATTACH]
Now I have the pattern isolated to 8 unique gray levels across the entire image. So you might say, wait, I need color. Well that's the next step.
What we now apply is a Gradient Map adjustment Layer which turn grayscale into a color. I suggest using the rainbow spectrum map that has 7 colors as I have shown:. I have opened the gradient editor so you can see all of the settings. (Note it is seven colors yet with editing the color map you can change it to 8, I just wanted to start with an easy example where you initially don't have to edit the gradient map and just use an existing gradient map)
[ATTACH=full]140491[/ATTACH]
Now you might say. How to I adjust the colors to whatever you want. That is simple done by editing the color to change the color of any of the color stops or even adding another color stop. The approach to do this is in the Adobe documentation and this is an example link on how to make adjustments:
https://helpx.adobe.com/photoshop/using/gradients.html#gradient-editor
Bottom line is that you can now indepently control the groups of colors to any color you desire.
Now there is another issue though. You also asked how do I differentiate the colors so they don't look mushed together. I will give a few pointers and leave it at that unless you end up with more questions
1) The eye sees contrast better than it sees colors. So you want adjacent colors to have a different color luminosity.
2) Using color theory on what colors look nice in the same image is also helpful
3) I you really want the chosen colors you can fake some higher contrast by selecting those colors, ceate a Layer Mask, and then set a color style to place a contrasting stroke around the color (eg black) or a drop shadow around those particular colors.
4) You can just try a variety of already supplied gradient map presets to see what they generate for you.
Hope this gives you some approaches to consider.
John Wheeler
