Balagesz tried it!!!
All my suppositions are correct: 16 colors can be used on any display mode, with the highest horizontal resolution, Nick clock are the pixel clock.
1.Two counters used. One 10 bit counter for the Nick clock (pixel clock). HSYNC clear it, then it is the horizontal position. Bit 9 of this activate the EXTCOL. Another 9 bit counter count the HSYNC, and cleared by the VSYNC. This is the vertical position. Bit 7,6,5,4 connected to EC3,2,1,0
2.More complex try out hardware: the horizontal counter used for RAM A9-A0, vertical counter for A18-A10. Every byte one pixel. Bit 3,2,1,0 go to EC3,2,1,0, and Bit 7=1 activate the EXTCOL.
3.Same test as the 1), but now on Iview image. You can see the rectangles using palette colors are changing colours, which are using BIAS colors are fixed. (Second pics: the external color RAM filled with random values at power on.)