Often a characteristic that can be written to will also allow you to read the current value of that descriptor. If you have a RGBW lamp it may be possible to read from a characteristic and receive the result [0, 255, 0, 0]. If your lamp is currently green, the data is probably formatted like [R, G, B, W]. If your lamp is red, it is probably formatted like [W, R, G, B]. In such a case you can try writing [255, 0, 0, 0] to see what it does.