By default, the new image have UNI-palette. It is universal palette, where exists ALL colors, but it's count is 256. When you image in 8bpp mode have UNI-palette - then you can use Effects for this image (as in 24bpp mode). Also, colors FC and BC in image (which have UNI-palette) you can select with special 'Spectral select'. For 'Spectral select' calling in palette window press 'JoyLeft'. In 'Spectral select': 1 and 3 select color, 4 and 6 select brightness OR 7 and 9 select one of gray colors.