in reply to Regexp experts, come to rescue!

One problem is you are performing the second and third regexps on the default variable '$_'. I.e. what your code is actually doing is this:
if ($MType =~ /-[45][15j]/ig || $_ =~ /[123][a46]u/ig || $_ =~ /7[9l][hb]/ig ) { print "\"red\""; } else { print "\"#CCFFCC\""; }
I would also offer '$MType =~ /^(-(41|55|jj)|(1a|24|36)u)$/' as a regexp that would match all the cases you list (and no others).
However, if they are the only cases you want to match, creating a hash and checking for the existence of keys would be more efficient:
my %matches; foreach (qw( -41 -55 -jj 1au 24u 36u)) { $matches{$_} = ''; } if (exists $matches{$MType}) { print "\"red\""; } else { print "\"#CCFFCC\""; }