// adjut these to fit your desired range
set neg -0.02
set pos 0.02

load ach_h.pdb -name p
load ach -type msms -name s
load ach.fld -name e

.p new -name all

.s new -name all
.s.all set color=red -range src=.e,val=-9999:$neg
.s.all set color=red:white -range src=.e,val=$neg:0.0
.s.all set color=white:blue -range src=.e,val=0.0:$pos
.s.all set color=blue -range src=.e,val=$pos:9999