If you already have objects in your document, have been using either the 81- or 168-color palettes, and you change to the 256-color palette in the Colors & Gradients dialog box, you will see a dialog box advising you that the colors may change. You're also asked to choose between "Preserve colors" or "Adjust colors".
What does this mean? When you change palettes, there is a possibility that the palette you're changing to (the 256-color palette) might not have the exact color or colors you've chosen from the palette you had been using. So ClarisDraw gives you a choice: either preserve the colors you've selected by remapping them into the 256-color palette, or change the colors to the closest available match already in the 256-color palette.
If you choose "Preserve colors," then ClarisDraw will first extract the colors you've already used in your document. It will then replace colors in the 256-color palette with the shades you've used. It will replace colors starting in the lower-right corner of the palette and work left and up. The colors that were originally in the 256-color palette will be permanently replaced with the new colors from your document.
If you select "Adjust Colors," ClarisDraw will go through the 256-color palette, find colors as close as possible to the ones you've already selected, and use them to fill the objects in your document.
Since there is color overlap between the three palettes, there is a possibility that the colors in your document might not change at all, regardless of which option you choose.
The options to preserve or adjust the colors are only available with the 256-color palette, because it is the only palette that is editable. If you switch from the 256-color palette to either the 81 or the 168-color palettes, you will only received a dialog telling you the colors "may be changed."