Increased colours to 256 but palette still all greys

[terrypin]

I opened a JPG in PSP 8 (yes, that ancient version) that I'd originally set in IrfanView grey scale. I now want to add a coloured rectangle, so I used Image > Increase Colour Depth > 256 Colours. I expected to see the palette offer those, but it didn't, remaining all greys, and I'm puzzled why not?

Meanwhile I've further increased to a million colours to add my coloured rectangle!

I'll load up PSP 2018 later to see if that's the same.

[LeviFiction]

There are 256 versions of grey. A greyscale image doesn't need a pallette, 0 is black, 255 is white, and every value in between is a form of grey. A 256 color palette increases the available colors by adding a pallette to the image to map the single byte index to. By default PSP retains the greyscale because that's technically what the image was already using. You can then edit the pallette to change one of the grey colors into any other 8-bit color.

[terrypin]

Thanks, understood.