I know how to code a solid bar GUI to represent a value, but how do i use multiple textures like the vanilla health and armor bars? Is it just a lot of this?

 

if (value > number && value1 < number {
               render one image
         else if (value > number && value1 < number {
               render one image next to other image 

Pretty much yeah, I remember having fun with editing it without events pre-1.6 (I actually overriden whole GuiIngame that time) and rendering was a loop of drawTexturedModalRect methods that were set by player's health and food etc.

This will be fun, lol.

https://github.com/MinecraftForge/MinecraftForge/blob/master/src/main/java/net/minecraftforge/client/GuiIngameForge.java#L258

Loops are your friend.