You can find the fade setting for xrefs on the expanded portion of the Reference panel on the Insert tab, in the Options dialog on the Display tab, or at the command line as XDWGFADECTL.
The default value is 70, the minimum is 0, and the maximum is 90. (Again, this keeps you from inadvertantly making an xref invisible.)
The icon at the left side of the slider bar enables you to quickly fading on and off without changing the fade value.
This setting is applied to every xref in the drawing -- you can't have different xrefs faded differently.
For more on external references, don't forget to sign up for the External References webcast next Tuesday!