Which of the following CSS3 Color Feature like RGB color but adds an alpha chann

Which of the following CSS3 Color Feature like RGB color but adds an alpha channel value to specify the opacity of the color?

2) Which of the following CSS3 Color Feature can be used as a macro for whatever the current color is?

3) Which of the following Color Format is a CSS colors can also be defined using the keyword rgb, followed by three numbers between 0 and 255, contained in parentheses and separated by commas, with no spaces between them?

4) The _______property is used to set the color of the text.

5) Which of the following Color Format is a CSS3 HSL value with a fourth value to set the alpha channel value for the color to define the opacity of the element?