All LCDs used on mobile phones use TN-Film LCD matrices which gives them 6-bit per color (or in human-readable numbers - 64 levels of red, green and blue). However, since programming in 18bit is a bit awkward, again, while the displays can show 18bit (3 x 6bit per color), or 262144 different colors, framebuffers are limited to 16-bit (or 65536 different colors).

So N900, just like any other mobile device featuring a LCD screen, is capable only of 16-bit color palette (or less). And it doesn't stop only on cell-phones, most, if not all of the laptops also have 18-bit screens, but since they have greater resolution and stronger hardware they could do away with working on 24-bit palette and using various dithering methods to give an illusion of fuller color fidelity. M/PVA and H/S-IPS screens that could display real 24-bit color palette are way too expensive and way too power consuming for mobile devices.