I've tested this on Quark 6.5 and it works fine, but have no access to 6.1. However the line is used in a book based on Quark 4, so I'm pretty sure it should work.
Try isolating just this line, create a new, empty doc in Quark and run it. It should create a new colour with no values assigned:
tell application "QuarkXPress 6.5"
tell document 1
set newcolorspec to (make color spec at beginning)
end tell
end tell
Anyone else know why this line should error? My mind's a blank...