diff --git a/map/traffic_manager.cpp b/map/traffic_manager.cpp
index a3d3dfb5c..bc6739433 100644
--- a/map/traffic_manager.cpp
+++ b/map/traffic_manager.cpp
@@ -518,9 +518,18 @@ void TrafficManager::DecodeFirstMessage()
     // store message in cache
     m_messageCache.insert_or_assign(message.m_id, message);
   }
-  // store message coloring in AllMwmColoring
-  // TODO trigger full cache processing if segments were removed or traffic has eased
-  traffxml::MergeMultiMwmColoring(message.m_decoded, m_allMwmColoring);
+  /*
+   * TODO detect if we can do a quick update:
+   *  - new message which does not replace any existing message
+   *  - coloring “wins” over replaced message:
+   *    - contains all the segments of the previous message (always true when location is the same)
+   *    - speed groups are the same or lower as in previous message (always true when all members of
+   *      traffic impact are unchanged or have worsened) – for this purpose, closure is considered
+   *      lower than any other speed group
+   *  In this case, run:
+   *      traffxml::MergeMultiMwmColoring(message.m_decoded, m_allMwmColoring);
+   *  Otherwise, set a flag indicating we need to process coloring in full.
+   */
 }
 
 void TrafficManager::ThreadRoutine()
@@ -779,6 +788,20 @@ void TrafficManager::OnTrafficDataUpdate()
 
   LOG(LINFO, ("Announcing traffic update, notifyDrape:", notifyDrape, "notifyObserver:", notifyObserver));
 
+  /*
+   * TODO introduce a flag to indicate we need to fully reprocess coloring, skip if it is false.
+   * The flag would get set when messages get deleted (including any clear/purge operations),
+   * or when a new message is added without indicating a simplified update in `DecodeFirstMessage()`.
+   * When we reprocess coloring in full (the block below), reset this flag.
+   */
+  {
+    std::lock_guard<std::mutex> lock(m_mutex);
+
+    m_allMwmColoring.clear();
+    for (const auto & [id, message] : m_messageCache)
+      traffxml::MergeMultiMwmColoring(message.m_decoded, m_allMwmColoring);
+  }
+
   /*
    * Much of this code is copied and pasted together from old MWM code, with some minor adaptations:
    *
@@ -819,6 +842,15 @@ void TrafficManager::OnTrafficDataUpdate()
 
         UpdateState();
 
+        /*
+         * TODO BUG: this leaves behind deleted segments
+         * If an update removes segments (but the MWM still has segments), some (the first added?)
+         * may be left behind but will disappear on the next update. This has been observed with
+         * TraFF Assessment Tool when updating a message with another one resulting in fewer
+         * segments. Unclear if routing is also affected. The number of segments in the affected
+         * MWM does not change between the botched and the working update, indicating the correct
+         * coloring was passed and the problem is on the receiving end.
+         */
         if (notifyDrape)
         {
           m_drapeEngine.SafeCall(&df::DrapeEngine::UpdateTraffic,
diff --git a/map/traffic_manager.hpp b/map/traffic_manager.hpp
index 6ffc19b56..337bd4813 100644
--- a/map/traffic_manager.hpp
+++ b/map/traffic_manager.hpp
@@ -394,6 +394,10 @@ private:
    * @brief Processes new traffic data.
    *
    * The new per-MWM colorings (preprocessed traffic information) are taken from `m_allMmColoring`.
+   * `m_allMwmColoring` is rebuilt from per-message colorings in `m_messageCache` as needed.
+   *
+   * This method is normally called from the traffic worker thread. Test tools may also call it from
+   * other threads.
    */
   void OnTrafficDataUpdate();
 
diff --git a/traffxml/traff_model.cpp b/traffxml/traff_model.cpp
index 54cf18943..bd639e48b 100644
--- a/traffxml/traff_model.cpp
+++ b/traffxml/traff_model.cpp
@@ -289,7 +289,7 @@ void TraffMessage::ShiftTimestamps()
     m_endTime.value().Shift(nowRef);
 }
 
-void MergeMultiMwmColoring(MultiMwmColoring & delta, MultiMwmColoring & target)
+void MergeMultiMwmColoring(const MultiMwmColoring & delta, MultiMwmColoring & target)
 {
   // for each mwm in delta
   for (auto [mwmId, coloring] : delta)
diff --git a/traffxml/traff_model.hpp b/traffxml/traff_model.hpp
index 05f5d7675..578fe3733 100644
--- a/traffxml/traff_model.hpp
+++ b/traffxml/traff_model.hpp
@@ -450,7 +450,7 @@ using TraffFeed = std::vector<TraffMessage>;
  * @param delta Contains the entries to be added.
  * @param target Receives the added entries.
  */
-void MergeMultiMwmColoring(MultiMwmColoring &delta, MultiMwmColoring & target);
+void MergeMultiMwmColoring(const MultiMwmColoring & delta, MultiMwmColoring & target);
 
 std::string DebugPrint(IsoTime time);
 std::string DebugPrint(Directionality directionality);