diff --git a/rfg_adc_plotter/gui/pyqtgraph_backend.py b/rfg_adc_plotter/gui/pyqtgraph_backend.py
index db94ae6..13cdf9c 100644
--- a/rfg_adc_plotter/gui/pyqtgraph_backend.py
+++ b/rfg_adc_plotter/gui/pyqtgraph_backend.py
@@ -170,6 +170,30 @@ def resolve_visible_aux_curves(aux_curves: SweepAuxCurves, enabled: bool) -> Swe
     return aux_1_arr, aux_2_arr
 
 
+def compute_background_subtracted_bscan_levels(
+    disp_fft_lin: np.ndarray,
+    disp_fft: np.ndarray,
+) -> Optional[Tuple[float, float]]:
+    """Pick robust color levels from positive residuals after background subtraction."""
+    if disp_fft_lin.size == 0 or disp_fft.size == 0:
+        return None
+    positive_mask = np.isfinite(disp_fft_lin) & (disp_fft_lin > 0.0)
+    if np.count_nonzero(positive_mask) < 2:
+        return None
+    vals = np.asarray(disp_fft[positive_mask], dtype=np.float64).reshape(-1)
+    vals = vals[np.isfinite(vals)]
+    if vals.size < 2:
+        return None
+    try:
+        vmin = float(np.nanpercentile(vals, 15.0))
+        vmax = float(np.nanpercentile(vals, 99.7))
+    except Exception:
+        return None
+    if not (np.isfinite(vmin) and np.isfinite(vmax)) or vmin >= vmax:
+        return None
+    return (vmin, vmax)
+
+
 def run_pyqtgraph(args) -> None:
     """Start the PyQtGraph GUI."""
     peak_calibrate_mode = bool(getattr(args, "calibrate", False))
@@ -1330,15 +1354,8 @@ def run_pyqtgraph(args) -> None:
 
             levels = None
             if active_background is not None:
-                try:
-                    p5 = float(np.nanpercentile(disp_fft, 5))
-                    p95 = float(np.nanpercentile(disp_fft, 95))
-                    span = max(abs(p5), abs(p95))
-                    if np.isfinite(span) and span > 0.0:
-                        levels = (-span, span)
-                except Exception:
-                    levels = None
-            else:
+                levels = compute_background_subtracted_bscan_levels(disp_fft_lin, disp_fft)
+            if levels is None:
                 try:
                     mean_spec = np.nanmean(disp_fft, axis=1)
                     vmin_v = float(np.nanmin(mean_spec))
diff --git a/tests/test_processing.py b/tests/test_processing.py
index 283aef4..5802ca0 100644
--- a/tests/test_processing.py
+++ b/tests/test_processing.py
@@ -9,6 +9,7 @@ from rfg_adc_plotter.constants import FFT_LEN, SWEEP_FREQ_MAX_GHZ, SWEEP_FREQ_MI
 from rfg_adc_plotter.gui.pyqtgraph_backend import (
     apply_working_range,
     apply_working_range_to_aux_curves,
+    compute_background_subtracted_bscan_levels,
     resolve_visible_aux_curves,
 )
 from rfg_adc_plotter.processing.calibration import (
@@ -29,6 +30,7 @@ from rfg_adc_plotter.processing.fft import (
     compute_distance_axis,
     compute_fft_mag_row,
     compute_fft_row,
+    fft_mag_to_db,
 )
 from rfg_adc_plotter.processing.normalization import (
     build_calib_envelopes,
@@ -180,6 +182,30 @@ class ProcessingTests(unittest.TestCase):
         self.assertTrue(np.allclose(visible[0], aux[0]))
         self.assertTrue(np.allclose(visible[1], aux[1]))
 
+    def test_background_subtracted_bscan_levels_ignore_zero_floor(self):
+        disp_fft_lin = np.zeros((4, 8), dtype=np.float32)
+        disp_fft_lin[1, 2:6] = np.asarray([0.05, 0.1, 0.5, 2.0], dtype=np.float32)
+        disp_fft_lin[2, 1:6] = np.asarray([0.08, 0.2, 0.7, 3.0, 9.0], dtype=np.float32)
+        disp_fft = fft_mag_to_db(disp_fft_lin)
+
+        levels = compute_background_subtracted_bscan_levels(disp_fft_lin, disp_fft)
+
+        self.assertIsNotNone(levels)
+        positive_vals = disp_fft[disp_fft_lin > 0.0]
+        self.assertAlmostEqual(levels[0], float(np.nanpercentile(positive_vals, 15.0)), places=5)
+        self.assertAlmostEqual(levels[1], float(np.nanpercentile(positive_vals, 99.7)), places=5)
+        zero_floor = disp_fft[disp_fft_lin == 0.0]
+        self.assertLess(float(np.nanmax(zero_floor)), levels[0])
+
+    def test_background_subtracted_bscan_levels_fallback_when_residuals_too_sparse(self):
+        disp_fft_lin = np.zeros((3, 4), dtype=np.float32)
+        disp_fft_lin[1, 2] = 1.0
+        disp_fft = fft_mag_to_db(disp_fft_lin)
+
+        levels = compute_background_subtracted_bscan_levels(disp_fft_lin, disp_fft)
+
+        self.assertIsNone(levels)
+
     def test_fft_helpers_return_expected_shapes(self):
         sweep = np.sin(np.linspace(0.0, 4.0 * np.pi, 128)).astype(np.float32)
         freqs = np.linspace(3.3, 14.3, 128, dtype=np.float64)