I'm genuinely unsure what you mean here. In SGR, 30-37 are foreground colors, 40-47 are background, and the brights are 90-97 for foreground and 100-107 for background. Even the second bright set is quite old, xterm calls them the aixcolor because they originated in IBM's AIX in the 1980s. I've never seen terminal software which doesn't support these and definitely never encountered one where the difference between foreground and background was negotiable. The concept of a color independent of being foreground and background can't be expressed in SGR codes.