First, you need to pull LightWorker(s) of the lights you want to change. (LightAPI has several helper methods to get certain LightWorkers in a zone or within range of a position.) Then, to add a modifier, call AddModifier on the LightWorker. For example, if you are applying the same modifier to multiple lights:

IEnumerable<LightWorker> workersInZone = LightAPI.GetLightWorkersInZone((0, 0, 0));
GetLightWorkersInZone<ILightModifier> lightMod = workersInZone.AddLightModifiers("red", 1.0f, true, LightPriority.Normal);

This will instantly enable all lights in the elevator zone, set intensity to 1, and change the color to red.

If you want to lerp light settings for individual lights, create a modifier with the current light settings as to not immediately apply new settings:

LightWorker worker = MyCoolWorkers[i]; // example list of LightWorkers
LG_Light light = worker.Light;
ILightModifier mod = worker.AddModifier(light.m_color, light.m_intensity, light.enabled);
// Go to MyCoolCoroutine...
while (time <= duration)
{
    time += Time.deltaTime;
    progress = Mathf.Clamp01(time / duration);
    mod.Color = Color.Lerp(startColor, endColor, progress);
    mod.Intensity = Mathf.Lerp(startIntensity, endIntensity, progress);
    yield return null;
}

All AddModifier methods return the created ILightModifier object(s), which provide methods to update or remove itself.

public interface ILightModifier
{
    // The color of this modifier.
    public Color Color { get; set; }

    // The intensity of this modifier.
    public float Intensity { get; set; }

    // The (light) enabled/disabled state of this modifer.
    public bool Enabled { get; set; }

    // The priority of the modifier. Highest priority takes affect; FIFO.
    public int Priority { get; }

    // Whether the modifier is currently active on the light.
    public bool Active { get; }

    // Sets a new light color, intensity, and enabled state for the modifier, if Active
    public void Set(Color color, float intensity, bool enabled) { }

    // Adds the full modifier back to the top of the stack.
    public bool Register(); 

    // Disables the modifier and removes it from the light.
    public void Remove();
}

Note: LightWorkers do not persist between levels.