When assigning the number of colours to a variable imagecolorstotal() returns 0.

$total = imagecolorstotal($image);
echo($total);

I was testing the greyscale example from here:

http://www.php.net/manual/en/function.imagecreatetruecolor.php

GD installed:

GD Support enabled
GD Version bundled (2.0 compatible)
FreeType Support enabled
FreeType Linkage with freetype
GIF Read Support enabled
JPG Support enabled
PNG Support enabled
WBMP Support enabled

Tests.  I tried this with:
gif - returns 256
jpeg (24bit colour) - returns 0
png 8bit - returns 256
png 24bit - returns 0

All the same image source image saved to the different formats from photoshop.

I've also tested the script on an older redat install and it works for all (well kinda - returns max 256, but that's an old release so who cares)  

details of the older machine:

PHP Version 4.1.2

GD Support enabled
GD Version 1.6.2 or higher
FreeType Support enabled
FreeType Linkage with TTF library
JPG Support enabled
PNG Support enabled
WBMP Support enabled

Ideally you'd want more than 256 colors anyway?  And maybe that's what supposed to happen?  

Saw a couple of non-english postings about it in google groups from what I could make out

I've also searched the bug database and could only find a passing mention as an aside to a different bug so......