Color Test

Simple test that should draw from left to right rectanges with red, green, blue and white color.

Useful for display driver diagnostics. The TFT_eSPI project describes quite nicely the different switches that can affect the colors in MIPI displays:

Different hardware manufacturers use different colour order
configurations at the hardware level.  This may result in
incorrect colours being displayed.
Incorrectly displayed colours could also be the result of
using the wrong display driver...
Typically displays have a control register (MADCTL) that can
be used to set the Red Green Blue (RGB) colour order to RGB
or BRG so that red and blue are swapped on the display.
This control register is also used to manage the display
rotation and coordinate mirroring. The control register
typically has 8 bits, for the ILI9341 these are:
Bit Function
7   Mirror Y coordinate (row address order)
6   Mirror X coordinate (column address order)
5   Row/column exchange (for rotation)
4   Refresh direction (top to bottom or bottom to top in portrait orientation)
3   RGB order (swaps red and blue)
2   Refresh direction (top to bottom or bottom to top in landscape orientation)
1   Not used
0   Not used
The control register bits can be written with this example command sequence:

   tft.writecommand(TFT_MADCTL);
   tft.writedata(0x48);          // Bits 6 and 3 set

0x48 is the default value for ILI9341 (0xA8 for ESP32 M5STACK)
in rotation 0 orientation.

Another control register can be used to "invert" colours,
this swaps black and white as well as other colours (e.g.
green to magenta, red to cyan, blue to yellow).

Source: https://github.com/Bodmer/TFT_eSPI/blob/master/examples/Test%20and%20diagnostics/Colour_Test/Colour_Test.ino

References