This is one of my plugins for ArtEffect2/3+.

Color to Gray does just what its name promises, the color channels can
be freely weighted.

 PERMSSN    UID  GID    PACKED    SIZE  RATIO     CRC       STAMP          NAME
---------- ----------- ------- ------- ------ ---------- ------------ -------------
[generic]                 7629   12428  61.4% -lh5- 4c2b Jul 18  1999 plugins2/ColorToGray.pss
[generic]                 2531    6523  38.8% -lh5- df2f Jul 25  1999 Color_to_Gray.guide
---------- ----------- ------- ------- ------ ---------- ------------ -------------
 Total         2 files   10160   18951  53.6%            Jul 25  1999