More than 256 colors on indexed mode.

I think the other responses are a bit confused about your intention. As far as I can tell, you want to reduce an image to a 256 colour version using the 15bit 32678 colours of the SNES, correct?

This is my workflow:

1. Download SnesGFX v2.62: https://www.smwcentral.net/?p=section&a=details&id=6523
2. Reduce your image to a max width of 256px and a max height of 224px (or set the Resize Image to 100% in the SnesGFX settings).
3. Open the image in SnesGFX. Choose the Format - any 8bit format will work. We are not interested in the bitmap file output, but rather in the 256 generated colour palette it generates.
4. Click Preview to check if it will work. Then click SAVE.

This will save the graphics format file as used in SNES development, and a colour palette PAL file.

This PAL file contains exact colour values based on the SNES 15bit 32768 colour pallette, and reduced to 8bit 256 colours for that particular image that was fed into it.

The problem with Photoshop: it refuses to load the generated PAL file, because PAL files come in different guises. Photoshop's support is limited to the Microsoft PAL variant, I believe.

While it is possible to convert colour palette files to ACT files that Photoshop understands, it involves obscure command line tools. Instead, just download the latest build of GIMP.

https://www.gimp.org/downloads/devel/

1) Open GIMP

2) Open your image from before.

3) Navigate to the PALETTE panel, and right-mouse click-->Import Palette.

4) Click PALETTE FILE, and select the generated PAL file. Click IMPORT

5) Choose IMAGE-->MODE-->INDEXED

6) Choose USE CUSTOM PALETTE and select your generated SnesGFX colour PAL file.

7) optional: enable dithering.

😎 click CONVERT

Done. Save as a PNG file.

SNES is not a color table. Would you really want to choose each colour from a scroll down table of 32768 colours? And the colours are always the same. No... it is a 15-bit RGB, with 5-bits per channel. Whatever tool you would use to convert to SNES 15-bit colour will likely take 24-bit RGB and truncate or dither. https://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_color_formats#15-bit_RGB

Indexed color is limited to 256 colors. Use 8 or 16 bit RGB.