You should use 32 bit color mode because many modern graphics cards don't support 24 bit color modes anymore. Both 24 bit color mode and 32 bit color mode use one byte per color component. 32 bit color mode uses an extra byte for alpha blending which however is not directly supported by DirectDraw.
Modifying the TurboPixel code to work with 32 bits shouldn't be a big deal.

[pascal]
result := pword(integer(LockedSurfaceDesc.lpsurface) + // surface pointer
y * LockedSurfaceDesc.lpitch + x * 4)^; [/pascal]

I believe adding 4 to the surface pointer instead of 3 will do the trick. I'm quite sure you won't have to change the way you compose your color values, so simply try the above.