So the brightness adjustment will only have an effect if the LED is not already at max brightness. So if you have a red led and the color in your graphic app is rgb(255, 0, 0) or rgb(1, 0, 0) depending how it shows then it is already at maximum brightness.
So you may want the light to be not at max brightness and put whatever desired “shaping” of the light source (E.g. brighter in the middle) in the image, so it can be darkened or brightened, when it’s more dark you will see more shape, and as it gets brighter it will become more flat looking.