What we know:
Nick have a external 4 bit color input (16 colors): EC0-EC3 pins. And one control wire /EXTC which indicates the presence of external color on EC0-EC3.
Then Nick internal can mix the internal and external color informations:
-external color displayed instead the internal
-external color displayed if the internal are in COL8-COL15 range
-external color displayed if the internal are in COL8-COL15 range OR the external color in COL0-COL7 range (EC3 low)
-external color displayed if the internal are in COL8-COL15 range OR the external color in COL0-COL3 or COL8-COL11 range (EC2 low)
The last 3 modes can be used for mixing playfield elements (internal graphics) with sprites (external graphics).
What we don't know
what will happen if send anything to these inputs. Probably only tested at the developers...
In 16 color graphics mode everything are logical. But what will happen in 2 or 4 colors mode?
I guess external 16 colors also can be used in 2 colors mode! Why? The Text 80 mode using hires 2 graphics with special settings, and 8 colors possible. Then I think very possible the Nick can read the external color information from the input pins at every pixel.
Firstly need some test electronics, which are send information to input pins, for example a simple counter of Nick clock will send a 00,01,...0E,0F. Then lets view which colors displayed in the screen.
My theory for the sprites:
The electronics will run on Nick clock (available on the bus). Will count the clock signals as pixel clock. The H and V sync also available on the bus, these can be used for reset the counters.
And if the counters in the sprite are then send the actual sprite pixel color to the EC0-EC3, and set EXT to low.
For store the sprite data probably some dual port SRAM are the optimal.
Then one sprite done
For the next, more sprites, mixing these, how sprites move (CPU update the coordinates, or the sprite unit move these?), etc
Probably will be good make the sprite unit with similar features with exiting ones (MSX, CPC+, etc?) the easy can be ported existing sprite games.
Another usage (with more powerfull hw): the whole screen will be as "sprite". If my theory good then possible use 16 colors with hires 2 resolution! Add horizontal scroll features, which are really missing from the Nick...
Also possible more powerfull Spectrum emulation, the whole Spectrum screen genarated as "external ULA" then not need attribute conversion!