NTSC Palette
There are in effect 128 different colours, as D0 of colour registers is unused. The image below shows the results of the 256 possible values that can be selected. Although each pair appears to be slightly different, this is an optical illusion. They appear in pairs because D0 is ignored and so (for example) colour $34 and colour $35 are identical. Colour number goes down the rows, and luminance across the columns (so, row 3, column 4 - both counting from 0 - is colour value $34 and appears red on NTSC systems).
PAL Palette
The PAL palette is a lot like the NTSC one. It also ignores the D0 bit for the luminance so there's only 128 "usable" values.
But thanks to the way it were implemented it only has 104 unique colors to chose from, the colors are also interleaved instead of being laid out nicely as with NTSC. So when you develop for PAL you should alter the values that you use for the palette.
SECAM Palette
The SECAM palette was reduced to a simple 3-bit RGB, containing only 8 colours (black, blue, red, magenta, green, cyan, yellow and white) by mapping the hue (COL3-COL0) to luminance and ignoring the hue setting:
COL3 D7 | COL2 D6 | COL1 D5 | COL0 D4 | RGB1 | Visible Colour | ||||||||
| $0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $A $B $C $D $E $F | 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 | 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 | 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 | 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 | 000000 2120FF F03C79 FF50FF 7FFF01 7EFFFF FFFF3F FFFFFF | ||||||||