$ cat foo.pl
#!/usr/bin/perl -w

use strict;
use warnings;

my $text = '<font color="blue"><i><b> <br>Some text</font>,  <br>an a';
$text =~ s~<font color=("|')?((\w|\s)+)("|')?>(.*?)<\/font>~\[color=$2\]$5\[/color\]~ig; 
print $text . "\n";

$ ./foo.pl
[color=blue]<i><b> <br>Some text[/color],  <br>an a
$