diff --git a/backend/src/neighbourhood.rs b/backend/src/neighbourhood.rs
index b13adc6..c4ad24c 100644
--- a/backend/src/neighbourhood.rs
+++ b/backend/src/neighbourhood.rs
@@ -260,8 +260,17 @@ impl Neighbourhood {
             let double_ended = map.directions[r] == Direction::BothWays;
             if let Some(polygon) = make_arrow(line, thickness, double_ended) {
                 let mut f = map.mercator.to_wgs84_gj(&polygon);
-                // TODO Which cell?
                 f.set_property("kind", "border_arrow");
+                match self
+                    .derived
+                    .as_ref()
+                    .unwrap()
+                    .render_cells
+                    .colors_per_border[&i]
+                {
+                    Color::Disconnected => f.set_property("cell_color", "disconnected"),
+                    Color::Cell(idx) => f.set_property("cell_color", idx),
+                }
                 features.push(f);
             }
         }
diff --git a/backend/src/render_cells.rs b/backend/src/render_cells.rs
index 8757fe4..001950e 100644
--- a/backend/src/render_cells.rs
+++ b/backend/src/render_cells.rs
@@ -1,13 +1,14 @@
-use std::collections::{HashSet, VecDeque};
+use std::collections::{HashMap, HashSet, VecDeque};
 
 use geo::{BooleanOps, BoundingRect, Coord, Densify, Euclidean, LineString, MultiPolygon, Rect};
 use utils::{Grid, LineSplit};
 
-use crate::{Cell, MapModel, Neighbourhood};
+use crate::{Cell, IntersectionID, MapModel, Neighbourhood};
 
 const NUM_COLORS: usize = 10;
 const RESOLUTION_M: f64 = 10.0;
 
+#[derive(Clone, Copy)]
 pub enum Color {
     Disconnected,
     Cell(usize),
@@ -18,6 +19,8 @@ pub struct RenderCells {
     pub polygons_per_cell: Vec<MultiPolygon>,
     /// Colors per cell, such that adjacent cells are colored differently
     pub colors: Vec<Color>,
+    /// Each border belongs to a cell; put its color here
+    pub colors_per_border: HashMap<IntersectionID, Color>,
 }
 
 impl RenderCells {
@@ -102,18 +105,20 @@ impl RenderCells {
         if true {
             finalize(
                 grid,
+                cells,
                 cell_colors,
                 bounds,
                 &MultiPolygon::new(vec![boundary_polygon]),
             )
         } else {
-            debug_grid(grid, cell_colors, bounds)
+            debug_grid(grid, cells, cell_colors, bounds)
         }
     }
 }
 
 fn finalize(
     main_grid: Grid<Option<usize>>,
+    cells: &Vec<Cell>,
     cell_colors: Vec<Color>,
     bounds: Rect,
     boundary: &MultiPolygon,
@@ -121,6 +126,7 @@ fn finalize(
     let mut result = RenderCells {
         polygons_per_cell: Vec::new(),
         colors: Vec::new(),
+        colors_per_border: HashMap::new(),
     };
 
     for (idx, color) in cell_colors.into_iter().enumerate() {
@@ -170,15 +176,25 @@ fn finalize(
             .polygons_per_cell
             .push(cell_polygon.intersection(boundary));
         result.colors.push(color);
+
+        for i in &cells[idx].borders {
+            result.colors_per_border.insert(*i, color);
+        }
     }
 
     result
 }
 
-fn debug_grid(grid: Grid<Option<usize>>, cell_colors: Vec<Color>, bounds: Rect) -> RenderCells {
+fn debug_grid(
+    grid: Grid<Option<usize>>,
+    cells: &Vec<Cell>,
+    cell_colors: Vec<Color>,
+    bounds: Rect,
+) -> RenderCells {
     let mut result = RenderCells {
         polygons_per_cell: Vec::new(),
         colors: Vec::new(),
+        colors_per_border: HashMap::new(),
     };
 
     for (idx, color) in cell_colors.into_iter().enumerate() {
@@ -205,6 +221,10 @@ fn debug_grid(grid: Grid<Option<usize>>, cell_colors: Vec<Color>, bounds: Rect)
 
         result.polygons_per_cell.push(MultiPolygon::new(squares));
         result.colors.push(color);
+
+        for i in &cells[idx].borders {
+            result.colors_per_border.insert(*i, color);
+        }
     }
 
     result
diff --git a/web/src/RenderNeighbourhood.svelte b/web/src/RenderNeighbourhood.svelte
index 5851e34..221ded4 100644
--- a/web/src/RenderNeighbourhood.svelte
+++ b/web/src/RenderNeighbourhood.svelte
@@ -98,8 +98,7 @@
     {...layerId("border-arrows")}
     filter={["==", ["get", "kind"], "border_arrow"]}
     paint={{
-      "fill-color": "cyan",
-      "fill-opacity": 0.5,
+      "fill-color": ["get", "color"],
     }}
   />
   <LineLayer
diff --git a/web/src/wasm.ts b/web/src/wasm.ts
index 7f23390..ba7bcc6 100644
--- a/web/src/wasm.ts
+++ b/web/src/wasm.ts
@@ -164,6 +164,15 @@ export interface RenderNeighbourhoodOutput {
         }
       >
     | Feature<Point, { kind: "border_intersection" }>
+    | Feature<
+        Polygon,
+        {
+          kind: "border_arrow";
+          cell_color: "disconnected" | number;
+          // Populated by setCellColors, not in the WASM API
+          color?: string;
+        }
+      >
     | Feature<
         MultiPolygon,
         {
@@ -204,7 +213,7 @@ function setCellColors(
   ];
 
   for (let f of gj.features) {
-    if (f.properties.kind != "cell") {
+    if (f.properties.kind != "cell" && f.properties.kind != "border_arrow") {
       continue;
     }
     if (f.properties.cell_color == "disconnected") {