Let's see:
I think what you need to do is use the BOX command and make a big rectangle. After you set the top, left, and bottom values where you want them, make the length of the bar change with the right value (and the health). Like this:
box left#, top#, right#, bottom#
As the health decreases, change the value accordingly by finding the max length of the bar and subtracting the amount you need.
percentageofhealth# = maxhealth# * healthrightnow#
right#=(maxlength# + left) * percentageofhealth#
As your health decreases you can change the color of the bar by using the INK command. (The box command creates a box in the color of the last ink command made.) That should do it, in theory.
Shining Blue Blade of Night