lcd.print("R :"+ GetFromSerial());
but i do not know why it can not work in lcd to display out the RED value
Quotebut i do not know why it can not work in lcd to display out the RED valueIt can. You MUST use TWO statements to print the "R :" part and the value. You can NOT use the addition operator HOPING that somehow it will magically be interpreted as a concatenation operator, just because that is what you want.