you should better use a dedicated module for that, but just for fun

use v5.12;
use warnings;


while ( <DATA> ) {
    chomp;
    say "'$_'";
    say $3 while                # print 3rd capture
      $_ =~ /(^|\s)                # starts with line's start or white
+space
             ("?)                  # capture optional double-quote ins
+ide (otherwise empty string) 
             ([^\2]*?)             # capture anything non-quote 
             \2                    # end with same quote 
             (?=(\s|$))            # look-ahead at following whitespac
+e or line's end
            /xg                    # g= loop _globally_ over all match
+es   x=allow multiline extended regex
}




__DATA__
50 0 "R0 G255 B0 A255" "Solid" 118 1 "R0 G0 B0 A255" "R0 G0 B0 A255" 0
70 0 "R0 G255 B255 A255" "Solid" 118 1 "R12 G12 B12 A255" "R12 G12 B12
+ A255" 0
[download]

output:

'50 0 "R0 G255 B0 A255" "Solid" 118 1 "R0 G0 B0 A255" "R0 G0 B0 A255" 
+0'
50
0
R0 G255 B0 A255
Solid
118
1
R0 G0 B0 A255
R0 G0 B0 A255
0
'70 0 "R0 G255 B255 A255" "Solid" 118 1 "R12 G12 B12 A255" "R12 G12 B1
+2 A255" 0'
70
0
R0 G255 B255 A255
Solid
118
1
R12 G12 B12 A255
R12 G12 B12 A255
0
[download]

update

added comments