I want to show an HP bar next to the units in the game. Here are my current HP bars.

Besides making the bars themselves prettier, what do you think of how the bars are positioned? The bars are drawn behind their units, since I don't want them obscuring the units. I don't want the unit to obscure the bar too much either, but that may not be as big of a deal since I'll also show an HP bar at the bottom along with other vitals when a unit is selected.

You could try positioning the health bar above the sprites, dont use different colours but only one red bar.
This red bar could have different lenghts for each different type of unit so the player can initially see the health difference of the units and better decide which one to take out first etc. The bar then shortens with each hit. So there is nothing of the health bar left in the end and the in defeated.

Just an idea though.

Make a transparent red sphere behind the unit and make it emty from top to bottom... and possibly change color too... :3

You could try positioning the health bar above the sprites, dont use different colours but only one red bar.
This red bar could have different lenghts for each different type of unit so the player can initially see the health difference of the units and better decide which one to take out first etc. The bar then shortens with each hit. So there is nothing of the health bar left in the end and the in defeated.

Hit points are represented by percentages, so I don't want the bars to be different lengths for each unit.
Not having a background for the bar could work though. Players may be able to tell what percentage is left from in a bar if its maximum length is as tall as its unit anyway.
I suppose I could just show the numerical HP value on the unit.