Provides support for system colours, in addition to literal colours, in text formatting.

The base class TRgb stores the 24-bit literal colour value using a TUint32. TLogicalRgb uses the MSB from iValue2 data member as an 8-bit index. The purpose of the index is to allow applications to use logical colours. If the index is zero, the value is not a logical colour; it is treated as an ordinary TRgb value. If the index is non zero (1-255), the colour should be translated by the application into a system colour. Indices 254 and 255 are reserved for the system foreground and background colours, respectively and should be translated into them. Translation from index to RGB colour occurs in the implementation of MFormParam::SystemColor().

All colours in the Text and Text Attributes API are stored using TLogicalRgb values and are initialised to either TLogicalRgb::ESystemForegroundColor or TLogicalRgb::ESystemBackgroundColor.

This system allows an application to set its text colours once, perhaps using the system-wide colour scheme, rather than having to set the colours each time a text object is created. It is also compatible with legacy code which expects TRgb rather than TLogicalRgb values: for example, the logical foreground and background colours have their bottom three bytes filled with black and white respectively so that code which expects TRgb values can still use them.