As someone who is colour blind, I had a huge amount of trouble telling the different health levels apart. So found health2.png, health3.png and health4.png and changed them from full circles, to one quarter, one half, and three quarter circles respectively. This makes them much easier to tell apart. I left full health and very low health alone.
If you want to use this yourself, download and place health2.png, health3.png and health4.png into the /game/images/HUD folder. I made these from the compressed version, but I imagine they should work fine on the normal version too.
EDIT: Extra feedback from a colour blind player. If, like me, you have a shit-ton of trouble reading the keypress quicktime events (the ones where it says "PRESS E !!" or "PRESS ENTER !!" etc), because the red text on that background is almost impossible to read, there is a fix/hack. Open events_screen.rpy in a text editor (I use notepad++), search for
text ("Press " + kd + " !!") color "#C80000" bold True size 40
and change #C80000 to something else. I found yellow ( #FFFF00 ) and white ( #FFFFFF ) both work fine.
If you're not using the compressed version, you'll need to use unren to extract the .rpy files so you can edit them.