I guess your using this library?
https://github.com/adafruit/Adafruit-ST7735-LibraryIf so, I've just had a look and where the function accept colour it's looking for a 16 bit value, such as
fillScreen(uint16_t color)
The colours such as RED are defined in the header file
#define ST7735_RED 0xF800
So you should be able to pass and colour you like.
It even includes a function to convert 8 bits per colour into the single 16 bit value
// Pass 8-bit (each) R,G,B, get back 16-bit packed color
uint16_t Adafruit_ST7735::Color565(uint8_t r, uint8_t g, uint8_t b) {
return ((r & 0xF8) << 8) | ((g & 0xFC) << 3) | (b >> 3);
}