Just set a line of coords to check every tick

"led_r": {
    behavior: behaviors.WALL,
    reactions: {
        "light": {"charge1":1},
        "liquid_light": {"charge1":1},
    },
    color: "#660000",
    colorOn: "#ff0000",
    category: "machines",
    tempHigh: 1500,
    stateHigh: ["molten_glass","molten_glass","molten_glass","molten_gallium"],
    conduct: 1,
    breakInto: "glass_shard"
},

"led_r": {
    behavior: behaviors.WALL,
    reactions: {
        "light": {"charge1":1},
        "liquid_light": {"charge1":1},
    },
    color: "#660000",
    colorOn: "#ff0000",
    category: "machines",
    tempHigh: 1500,
    stateHigh: ["molten_glass","molten_glass","molten_glass","molten_gallium"],
    conduct: 1,
    breakInto: "glass_shard"
},

elements.led = {
    behavior: behaviors.WALL,
    reactions: {
        "light": {"charge1":1},
        "liquid_light": {"charge1":1},
    },
    color: "#008000",
    colorOn: "#FFFFFF",
    category: "machines",
    tempHigh: 1500,
    stateHigh: ["molten_glass","molten_glass","molten_glass","molten_gallium"],
    conduct: 1,
    breakInto: "glass_shard",
    tick: function(pixel){
    if (!pixel.color === pixel.colorOn){
    pixel.colorOn = pixel.color
    }
    }
}

elements.led = {
    behavior: behaviors.WALL,
    reactions: {
        "light": {"charge1":1},
        "liquid_light": {"charge1":1},
    },
    color: "#008000",
    colorOn: "#FFFFFF",
    category: "machines",
    tempHigh: 1500,
    stateHigh: ["molten_glass","molten_glass","molten_glass","molten_gallium"],
    conduct: 1,
    breakInto: "glass_shard",
    tick: function(pixel){
    if (!pixel.color === pixel.colorOn){
    pixel.colorOn = pixel.color
    }
    }
}