Horzline attribute

Hmm.. yep. That's a side effect of this not quite being the right way to do it. Will probably need to wait for horzline to support colours before you can set everything the way you need.

Or, if you want to extend the temporary solution even futher, you could draw a dashed line at 50 with a very creative script like:
[drawundefined=gap; color=Red]
if(mod(barnumber,4)<3,50,undefined);
..but again, this is just looking for creative ways around something that isn't really possible in BullCharts at the moment.

Unfortunately not at the moment.

The best way to work around this at the moment is to return the line as a calculated value instead of using the horzline attribute. (it means they don't appear in the horz line list, and can't be as readily moved, but will probably do what you're after)

Eg:

[color=red]
20;
[color=green]
80;