The datasheet for ILI9486 advertises 3-bit / 8 color mode for 3/4 wire SPI. I'd love to play with it but couldn't find any references on how to actually use it. I've tried a few things and nothing seems to work 
Did anyone figure out how to do it? I'll appreciate any hints! Thanks!
I just tried this on my Waveshare shield, it worked pretty much as described in the datasheet. Just 'write' to register 0x39 to enter 8 color mode (they call it idle mode), 0x38 to exit. No parameters, just sending the register number to the chip does the job.
Boiled down, it's basically
SPI.startTransaction(...)
Set chip select LOW
set DC LOW,
SPI transfer(0); Â Â Â Â // Â Even though registers are listed as 8 bit, transfer 16 bits!
SPI transfer(0x39);
Set Chip select high
SPI end transaction.
It's basically the same as inverting the display, just different registers.
GitHub - MHotchin/Waveshare4InchTftShield: Graphics and touchscreen drivers for Waveshare 4" shield is a driver for a particular shield, see the 'invertDisplay' command.
Oh, and this is a display mode - the data commands are all the same, you still write full color information to the display, and when you exit Idle mode, you get full color expression again.