Исправил обрезание красной компоненты цвета #1
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В определении функции void drawLetter(int index, uint8_t letter, int offset, int letterColor)
letterColor двухбайтный, соответственно, когда в него передаёшь красный цвет 0xff0000, обрезается всё кроме младших двух байтов. В случае красного цвета, цвет обрезается до 0x0000, т.е. до черного.