143 tells the computer that the graphics colour to be used is to be logical colour 3 and that this is to appear no matter what was on the screen under the new line or triangle. Values other than 0, for the first number, have other effects. For example, a value of 4, as in GCOL 4,0 has the effect of drawing a line which is the ‘inverse’ logical colour to the colour that it is currently crossing over. In two colour MODE s the inverse of logical colour 0 is logical colour 1. In four colour modes the following applies. With ‘default’ actual colour of black, red, yellow and white the inverse colours would be white, yellow, red and black.
In MODE 2 , the 16 colour MODE , all steady colours translate to flashing colours and vice versa as the next table shows for the default colours. Logical colour 0 1 2 3 Inverse 3 2 1 0 Logical colour 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Default displayed colour Black Red Green Yellow Blue Magenta Cyan White Flashing black/white Flashing red/cyan Flashing green/magenta Flashing yellow/blue Flashing blue/yellow Flashing magenta/green Flashing cyan/red Flashing white/black Inverse Flashing white/black Flashing cyan/red Flashing magenta/green Flashing blue/yellow Flashing yellow/blue Flashing green/magenta Flashing red/cyan Flashing black/white White Cyan Magenta Blue Yellow Green Red Black