I'm pretty sure not. Animatable Scene attributes all have the button with three dots, used to add a keyframe or access a controller script. The light radius (and shadows) dialog does not include this button. Generally light brightness is adjusted as you suggested, by modifying the colour, and this is an animatable attribute. For example, to define a dim red light the colour hue should be red and the brightness bar nearly at the black end. Although this looks wrong in the colour chooser preview, appearing a really dark burgundy, in the render the light will look dimly glowing red, like an ember. For lights, the vertical brightness bar in the colour chooser basically works like a dimmer from the middle position to the bottom (from the middle to the top the colour will bleach to white).