CLEARPALETTE does nothing. It has been kept for compatibility with old programs, but its function is no longer needed.
Historically, early versions of Microsoft Windows supported video displays that were limited to showing at most 256 colors at one time, although each of the colors used a full 24-bit range. The set of 256 possible colors was called the "palette" and could be changed depending on which colors you wanted to use. Whenever FMSLogo used a new color (for example, when SETPENCOLOR was run), it would add the color to this palette. Once all of the 256 colors in the palette were set, a program could not draw a new color, even if some of the colors in the palette were not in active use. The CLEARPALETTE command used to remove all colors from the palette so that the palette could be refilled with new colors.
Display devices, even old ones, no longer have a limited palette of colors, so CLEARPALETTE is no longer needed.