Creating your own color picker and then converting the photo

[lizardwizard97]

Hi, it is possible to create your own color picker and then convert the photo so that it is ONLY in the colors you have chosen (in addition, based on the photo below)?

[c.pfaffenbichler]

You can create a Color Table (up to 256 colors) and use that in Indexed Mode. 

What is the purpose for this? Actual spot colors output? 

[Chuck Uebele]

While indexed color is your best option, there are some issues with it. What you may want a particular color assigned to another color, Photoshop might determine that one of your other colors are a closer match. You might want a bright yellow, but it might assign a green or brown.