Add blog post about changes to light color mode (#2055)

Co-authored-by: Franck Nijhof <git@frenck.dev>

blog/2024-01-23-light-color-mode-changes.md

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+---
+author: Erik Montnémery
+authorURL: https://github.com/emontnemery
+title: "Changes to light color mode when lights display an effect"
+---
+
+## Background
+
+The primary reason for introducing light color modes was that a light's state should not be ambiguous. As an example, a light which supports color and white with adjustable color temperature must be in either color mode `hs` (for example) or `color_temp`.
+
+However, effects complicate this because when the same light is rendering an effect, none of the `hs_color`, `color_temp`, or `brightness` state attributes may be meaningful.
+
+## Changes
+
+### Requirements on `color_mode` are less strict when a light is rendering an effect
+
+More restrictive color modes than what's otherwise supported by the light are allowed when an effect is active:
+- A light which supports colors is allowed to indicate color modes `on_off` and `brightness` when controlled by an effect
+- A light which supports brightness is allowed to indicate color mode `on_off` when controlled by an effect.
+
+For example, a light which has its supported_color_modes set to `{"hs", "color_temp"}` is allowed to set its `color_mode` to `on_off` when rendering an effect which can't be adjusted and to `brightness` when rendering an effect which allows brightness to be controlled.
+
+### A special effect `EFFECT_OFF` which means no effect / turn off effect has been added
+
+There was previously no standard way for a light which supports effects to show that no effect is active.
+This has been solved by adding an the pre-defined effect `EFFECT_OFF` to indicate no effect is active.
+
+More details can be found in the [documentation](/docs/core/entity/light#color-modes) and in [architecture discussion #960](https://github.com/home-assistant/architecture/discussions/960).