I'd like to be able to change the fog color when swimming in a custom liquid. Lava and water are done this way:

 int i = ActiveRenderInfo.getBlockIdAtEntityViewpoint(this.mc.theWorld, entityliving, par1);

        if (this.cloudFog)
        {
            Vec3 vec33 = worldclient.getCloudColour(par1);
            this.fogColorRed = (float)vec33.xCoord;
            this.fogColorGreen = (float)vec33.yCoord;
            this.fogColorBlue = (float)vec33.zCoord;
        }
        else if (i != 0 && Block.blocksList[i].blockMaterial == Material.water)
        {
            this.fogColorRed = 0.02F;
            this.fogColorGreen = 0.02F;
            this.fogColorBlue = 0.2F;
        }
        else if (i != 0 && Block.blocksList[i].blockMaterial == Material.lava)
        {
            this.fogColorRed = 0.6F;
            this.fogColorGreen = 0.1F;
            this.fogColorBlue = 0.0F;
        }

Its already there. Just a few code lines above..