@jeffreyrdavis75 Thanks for clarifying what you wanted.
Since f gets called several times with the same values then it could be more efficient as, say,
f:=proc(x) option remember,system; x^3+3^x; end proc
though the above example wasn't terribly slow. I didn't bother.
If you really like the subdued colors in one of your question's images then something close can be obtained with minor adjustment to options and the color scheme functions.
f := x -> x^3 + 3^x:
plots:-densityplot(1, a=-17..21.5, b=-36..36,
(x,y,z)->`if`(Re(f(x+y*I))<17 and Im(f(x+y*I))<0,1.0,0.5),
axes=normal, axesfont=["Helvetica",8], xtickmarks=spacing(5));