I want to make the game look black and white temporarily, like with the old Super Secret Settings, how would I do this?

You can do that by calling ClientRegistry.registerEntityShader(PlayerEntity.class, new ResourceLocation("minecraft:shaders/post/desaturate.json")) in your FMLClientSetupEvent handler. This will desaturate the game partially. If you want full grayscale, you need to copy the shader JSON file, edit the desaturation factor and change the ResourceLocation to the new path. Be aware however that this will only grayscale the world, not the GUIs.

27 minutes ago, XFactHD said:

You can do that by calling ClientRegistry.registerEntityShader(PlayerEntity.class, new ResourceLocation("minecraft:shaders/post/desaturate.json")) in your FMLClientSetupEvent handler. This will desaturate the game partially. If you want full grayscale, you need to copy the shader JSON file, edit the desaturation factor and change the ResourceLocation to the new path. Be aware however that this will only grayscale the world, not the GUIs.

Thanks for the reply, seems like almost what I'm looking for, but the effect looks like it's permanent as I'm binding it to the player, is there a way I can make it toggleable?

You can probably call ClientRegistry.registerEntityShader() in a KeyBinding handler and call it with the correct ResourceLocation to activate it and with null to deactivate it. For the change to take effect you can just set the renderViewEntity again with

if (Minecraft.getInstance().getRenderViewEntity() instanceof PlayerEntity)
{
    Minecraft.getInstance().setRenderViewEntity(Minecraft.getInstance().getRenderViewEntity);
}

This will only set the renderViewEntity if it wasn't changed by another mod or MC itself to not be a player.