Colors using Adafruit_ST7735.h

Using Arduino Displays
4

You can look up standard 24-bit colours. i.e. 8-bits for RED, 8-bits for GREEN, ...
You convert them to to 16-bits i.e. 5-bits for RED, 6-bits for GREEN, 5-bits for BLUE:

#define RGB(r, g, b) (((r&0xF8)<<8)|((g&0xFC)<<3)|(b>>3))

#define GREY      RGB(127, 127, 127)
#define DARKGREY  RGB(64, 64, 64)
#define TURQUOISE RGB(0, 128, 128)
#define PINK      RGB(255, 128, 192)
#define OLIVE     RGB(128, 128, 0)
#define PURPLE    RGB(128, 0, 128)
#define AZURE     RGB(0, 128, 255)
#define ORANGE    RGB(255,128,64)

Since this is a common thing to do in TFT programs, the libraries tend to have suitable methods:
e.g. Adafruit_ST7735.h

  uint16_t Color565(uint8_t r, uint8_t g, uint8_t b);

e.g. MCUFRIEND_kbv.h

	uint16_t color565(uint8_t r, uint8_t g, uint8_t b) { return ((r & 0xF8) << 8) | ((g & 0xFC) << 3) | ((b & 0xF8) >> 3); }

David.