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You subclass SlotItemHandler. For example, one of these two: https://github.com/Draco18s/ReasonableRealism/tree/master/src/main/java/com/draco18s/hardlib/api/internal/inventory Note that SlotOutput will not allow any item to be inserted what so ever while OutputItemStackHandler is a wrapper around another handler that prevents input through the wrapper.

I have an on-going project where I use a standard .obj model and have the code create copies of the .obj, .mtl and a .png for particles etc, which includes editing each individual block's colour. This might be well over-the-top depending on how you want to render this model. Is the colour permanent? I mean, it does not change? If so, you would likely only ever need to create a class that implements IBlockColor, override colorMultiplier, and register the class with the wanted block using Minecraft::getBlockColors(). I do this for Items here. Do note that it wants an integer. either convert your rgb values to one, or create a java.awt.Color with these values, and call getRGB(). (Note, you can also use the constant colours like Color#GREEN etc)

You can send client's HP on connect and process damage on both server and client side, or send updated HP to client when damaged/healed. For last mob's HP bar you can create a field in capability and assign HP to it in attack event (and sync, too). Also, on client side track last attacked entity and save it to capability/variable too. Then, when you render entity, check if renderingEntity == lastAttackedEntity, and if it's true, render HP bar above it.