Hello,
This topic is a bit old, so you probably found the solution in a meanwhile, but this can help other people:
void setBackgroundColor(uint16_t color = COLOR_BLACK);
Nevertheless, this does not work with every methods. for example, the following code writes a text in yellow and white, but draws in ... black and white:
tft.setBackgroundColor(COLOR_YELLOW);
tft.setFont(Terminal6x8);
tft.drawText(10, 10, "Draw");
delay(1000);
tft.drawBitmap(0, 0, tux, 180, 220, COLOR_WHITE);
delay(1000);