Adding HUE function to graphics library

For some reason, TFT libraries support setting colors given by RGB either in 255, 255, 255 mode or using Color565, where red and blue are given als 5-bit values (0-31) and green as 6-bit value (0-63). To convert a given number to a color with maximum saturation, it has to be converted. This sketch shows you a simple way how this can be done:

#define cs   10
#define dc   9
#define rst  8

#include <TFT.h>
TFT tft = TFT(cs, dc, rst);

void setup() {
  tft.begin();
  tft.setRotation(3);
  tft.fillScreen(0);
  for (int i = 0; i < 32; i++) {
    int x = (i % 8) * 20; //  % 4;
    int y = (i / 8) * 32; // i / 4;
    int c = hue(i);
    tft.fillRect(x, y, 18, 30, c);
    tft.setTextColor(0);
    tft.setCursor(x+1, y+1);
    tft.print(i);
  }
}

void loop() {}

word hue(int z) {
  int M = 127;
  float h = z * M * 6 / 33.0;
  int g = max(-abs(h - 2 * M) + 3 * M - M, 0);
  int r = max(+abs(h - 3 * M) + 0 * M - M, 0);
  int b = max(-abs(h - 4 * M) + 3 * M - M, 0);
  return tft.Color565(r, g, b);
}

Uploading this sketch will give you exactly the picture shown above.

You also might try this sketch:

#define cs   10
#define dc   9
#define rst  8

#include <TFT.h>
TFT tft = TFT(cs, dc, rst);
int x = 0;
int y = 0;
void setup() {
  tft.begin();
  tft.setRotation(3);
  tft.fillScreen(0);
  tft.setTextSize(2);
  for (int i = 0; i < 32; i++) {
    int c = hue(i);
    tft.setTextColor(c);
    tft.setCursor(x, y);
    tft.print(i);
    if (i < 10)
      x = x + 15;
    else x = x + 27;
    if (x > 140) {
      x = 0;
      y = y + 24;
    }
  }
}

void loop() {}

word hue(int z) {
  int M = 127;
  float h = z * M * 6 / 33.0;
  int g = max(-abs(h - 2 * M) + 3 * M - M, 0);
  int r = max(+abs(h - 3 * M) + 0 * M - M, 0);
  int b = max(-abs(h - 4 * M) + 3 * M - M, 0);
  return tft.Color565(r, g, b);
}