Whoa guys, you have contributed a fair number of inaccuracies to this discussion, to whit:
320x200 games use either "rgb hue" for example "microsoft decathlon" or "640x200 hue" for example "gamma force".
No, both use 320x200 mode, the difference is the underlying colors they use.
My question is how hue was handled in the past? how does a monitor know whether it has to use "rgb hue" or "640x200 hue"?
The CGA card was generating the color based on the OSC signal coming from the ISA bus. On IBM PC/XT/AT machines, there was a hue control pot on the motherboard that tweaked the OSC signal going to the card. A real CGA card has no color controls of its own of any kind.
For 320x200 composite mode, it is simply a function of the monitor type - a rgb monitor will display it as rgb, a composite monitor as composite; there is no way for software to select one or the other. Only dual-function Composite/RGB monitors such as the Commodore 1084, with the right kind of cable (which probably would have to be manually soldered together), could display both; but you would still have to select the mode by toggling a hardware switch on the monitor.
For a modern TV set with SCART input (read: a European set), you could probably come up with a CGA-to-SCART cable which will switch the TV between RGB and composite input modes, again with a toggle switch and some external voltage source (battery, PC gameport, wall wart etc.) for the mode selection.
At least one variety of the Commodore 1084 had a 9-pin DE-9 port that could accept a digital or analog RGB signal. CGA sends out a digital signal. Since a SCART cable supports only analog RGB, you would need custom cable with a digital to analog conversion circuit. Software can select in 320x200 modes whether it displays in color or b&w on a composite monitor by selecting the 3rd palette/BIOS Mode 5.
It's a bit strange to think of mode 6 (monochrome) as having a palette, but you can change its foreground and background color. DOSBox's composite palette is designed to work with mode 6, foreground color 7 and 15 (white and intense white) on a black background, any other colors programmed into the palette registers on the CGA crtc could produce inaccurate composite colors.
CGA does not have palette registers, it has a color select register and palette select bit. Mode 6/640x200 has a palette of one color, selectable from 16 colors. If a color other than one of the whites is used, then it will produce whatever colors it produces. No game is known to use an off-white color. CGA cannot select the background color in this mode, it must be black.