You can achieve the result you want this way:
#mytext{
color: red;
opacity: 0.5;
}
Note that opacity will affect the whole element, not just the text, so for example, if the #mytext element had a background color, that would also receive the opacity value of 0.5
However, I agree with Dai, using color names instead of hex or rgb codes isn’t something you should rely on too much. It’s an ugly color palette to work with.