DonLPI is correct (thank you for helping!) that attribute 45 controls dithering. However, if you do not specify that attribute, the driver automatically turns it ON for 18bit panels so the colors look correct by default. We allow it to be overridden if you have a panel that has a better dithering capability than the chipset.
Additional information- the chipsets do spatial dithering by (in general) throwing out the least significant bits in the color information but then making semi-intelligent decisions on rounding up or rounding down the color for adjacent pixels.
If you are having color issues, you may have the LVDS data lines misrouted on the board or your cable.