diff --git a/src/time_slice.rs b/src/time_slice.rs
index bf6008d..cb1eed2 100644
--- a/src/time_slice.rs
+++ b/src/time_slice.rs
@@ -84,6 +84,17 @@ pub(crate) struct Event {
     weights: Vec<Vec<f64>>,
 }
 
+/// Output of a single event's inference pass — ready to apply back to shared state.
+struct EventOutput {
+    /// New per-team/per-item likelihoods (same shape as `event.teams`).
+    likelihoods: Vec<Vec<Gaussian>>,
+    evidence: f64,
+    /// (agent index, new skill likelihood) pairs for the sequential apply step
+    /// that updates `SkillStore`. Computed while holding `&SkillStore` so the
+    /// caller only needs `&mut SkillStore` when writing back.
+    skill_updates: Vec<(Index, Gaussian)>,
+}
+
 impl Event {
     pub(crate) fn iter_agents(&self) -> impl Iterator<Item = Index> + '_ {
         self.teams
@@ -115,6 +126,51 @@ impl Event {
             })
             .collect::<Vec<_>>()
     }
+
+    /// Compute the inference update for this event, returning an `EventOutput`
+    /// that describes the mutations to apply. Takes only shared references so
+    /// it can run inside a parallel closure.
+    fn compute<T: Time, D: Drift<T>>(
+        &self,
+        skills: &SkillStore,
+        agents: &CompetitorStore<T, D>,
+        p_draw: f64,
+    ) -> EventOutput {
+        let mut arena = ScratchArena::new();
+        let teams = self.within_priors(false, false, skills, agents);
+        let result = self.outputs();
+        let g = Game::ranked_with_arena(teams, &result, &self.weights, p_draw, &mut arena);
+
+        // Pre-compute new skill likelihoods while we still hold &skills.
+        let mut skill_updates: Vec<(Index, Gaussian)> = Vec::new();
+        for (t, team) in self.teams.iter().enumerate() {
+            for (i, item) in team.items.iter().enumerate() {
+                let old_skill_likelihood = skills.get(item.agent).unwrap().likelihood;
+                let new_item_likelihood = g.likelihoods[t][i];
+                let new_skill_likelihood =
+                    (old_skill_likelihood / item.likelihood) * new_item_likelihood;
+                skill_updates.push((item.agent, new_skill_likelihood));
+            }
+        }
+
+        EventOutput {
+            likelihoods: g.likelihoods,
+            evidence: g.evidence,
+            skill_updates,
+        }
+    }
+
+    /// Apply an `EventOutput` back onto this event's mutable item likelihoods
+    /// and evidence. The `SkillStore` updates are applied separately by the
+    /// caller to avoid conflicting borrows.
+    fn apply_output(&mut self, output: &EventOutput) {
+        self.evidence = output.evidence;
+        for (t, team) in self.teams.iter_mut().enumerate() {
+            for (i, item) in team.items.iter_mut().enumerate() {
+                item.likelihood = output.likelihoods[t][i];
+            }
+        }
+    }
 }
 
 #[derive(Debug)]
@@ -266,28 +322,79 @@ impl<T: Time> TimeSlice<T> {
     }
 
     pub fn iteration<D: Drift<T>>(&mut self, from: usize, agents: &CompetitorStore<T, D>) {
-        for event in self.events.iter_mut().skip(from) {
-            let teams = event.within_priors(false, false, &self.skills, agents);
-            let result = event.outputs();
+        if from > 0 || self.color_groups.is_empty() {
+            // Initial pass (add_events) or no color groups yet: simple sequential sweep.
+            for event in self.events.iter_mut().skip(from) {
+                let teams = event.within_priors(false, false, &self.skills, agents);
+                let result = event.outputs();
 
-            let g = Game::ranked_with_arena(
-                teams,
-                &result,
-                &event.weights,
-                self.p_draw,
-                &mut self.arena,
-            );
+                let g = Game::ranked_with_arena(
+                    teams,
+                    &result,
+                    &event.weights,
+                    self.p_draw,
+                    &mut self.arena,
+                );
 
-            for (t, team) in event.teams.iter_mut().enumerate() {
-                for (i, item) in team.items.iter_mut().enumerate() {
-                    let old_likelihood = self.skills.get(item.agent).unwrap().likelihood;
-                    let new_likelihood = (old_likelihood / item.likelihood) * g.likelihoods[t][i];
-                    self.skills.get_mut(item.agent).unwrap().likelihood = new_likelihood;
-                    item.likelihood = g.likelihoods[t][i];
+                for (t, team) in event.teams.iter_mut().enumerate() {
+                    for (i, item) in team.items.iter_mut().enumerate() {
+                        let old_likelihood = self.skills.get(item.agent).unwrap().likelihood;
+                        let new_likelihood =
+                            (old_likelihood / item.likelihood) * g.likelihoods[t][i];
+                        self.skills.get_mut(item.agent).unwrap().likelihood = new_likelihood;
+                        item.likelihood = g.likelihoods[t][i];
+                    }
                 }
-            }
 
-            event.evidence = g.evidence;
+                event.evidence = g.evidence;
+            }
+        } else {
+            self.sweep_color_groups(agents);
+        }
+    }
+
+    /// Full event sweep using the color-group partition. Colors are processed
+    /// sequentially; within each color the inner loop is parallel under rayon.
+    fn sweep_color_groups<D: Drift<T>>(&mut self, agents: &CompetitorStore<T, D>) {
+        // We need &self.skills (immutable) and &mut self.events (mutable) at the
+        // same time. Rust allows this because they are distinct struct fields.
+        // The parallel closure captures &self.skills and &self.p_draw by shared
+        // ref; it returns owned EventOutput values that we apply sequentially.
+        for color_idx in 0..self.color_groups.groups.len() {
+            if self.color_groups.groups[color_idx].is_empty() {
+                continue;
+            }
+            let range = self.color_groups.color_range(color_idx);
+
+            // Compute phase — parallel under rayon, sequential otherwise.
+            // Borrows: &self.skills and &agents are shared refs captured by the closure;
+            // &mut self.events[range] is the mutable slice for par_iter_mut.
+            let p_draw = self.p_draw;
+            let skills: &SkillStore = &self.skills;
+
+            #[cfg(feature = "rayon")]
+            let outputs: Vec<EventOutput> = {
+                use rayon::prelude::*;
+                self.events[range.clone()]
+                    .par_iter()
+                    .map(|ev| ev.compute(skills, agents, p_draw))
+                    .collect()
+            };
+
+            #[cfg(not(feature = "rayon"))]
+            let outputs: Vec<EventOutput> = self.events[range.clone()]
+                .iter()
+                .map(|ev| ev.compute(skills, agents, p_draw))
+                .collect();
+
+            // Apply phase — sequential: write skill likelihoods back to self.skills,
+            // then update per-event item likelihoods and evidence.
+            for (ev, output) in self.events[range].iter_mut().zip(outputs.iter()) {
+                for &(agent, new_skill_lhood) in &output.skill_updates {
+                    self.skills.get_mut(agent).unwrap().likelihood = new_skill_lhood;
+                }
+                ev.apply_output(output);
+            }
         }
     }