Sorry, but you can't assign line colors, etc, like that 🙂
With gplot, you set up the symbol statements with a number (n), such as symbol1 - symboln...
symbol1 v=plus ci=black i=join;
symbol2 v=plus ci=green i=join;
symbol3 v=plus ci=red i=join;
symbol4 v=plus ci=blue i=join;
proc gplot data=foo;
plot y*x=by-Variable;
Then the values of by-Variable (or whatever the variable name you use) are assigned to the symbol statements, in alpha/numeric order (there are a few caveats & tricks, but I won't get into them unless you bump into the problems that would require them!)