A gradient needs at least two colours to display. You can use hexadecimal values #efefef, #ff0000, named HTML colour entities goldenrod, pink, RGB values rgb(0,0,0) or rgba(0,0,0,0.5) or any other colour stop value that's supported in CSS linear-gradient().
#efefef, #ff0000
goldenrod, pink
rgb(0,0,0) or rgba(0,0,0,0.5)
linear-gradient()