Provide correct slope for color ratios with SpectralGradient.from_color

sbpy/spectroscopy/core.py

@@ -179,14 +179,14 @@ def from_color(cls, wfb, color):
 
         lambda_eff = SpectralGradient._lambda_eff(wfb)
 
-        try:
-            # works for u.Magnitudes and dimensionless u.Quantity
+        if isinstance(color, u.Magnitude):
+            alpha = u.Quantity(-color, u.dimensionless_unscaled)
+        elif u.Quantity(color).unit.is_equivalent(u.mag):
+            alpha = (-color).to(u.dimensionless_unscaled, u.logarithmic())
+        else:
             alpha = u.Quantity(color, u.dimensionless_unscaled)
-        except u.UnitConversionError:
-            # works for u.mag
-            alpha = color.to(u.dimensionless_unscaled, u.logarithmic())
 
-        dw = lambda_eff[0] - lambda_eff[1]
+        dw = lambda_eff[1] - lambda_eff[0]
         S = ((2 / dw * (alpha - 1) / (alpha + 1))
              .to(u.percent / hundred_nm))
 
@@ -446,7 +446,7 @@ def fit(self, spec):
 
         Examples
         --------
-        >>> spec_model = SpectralModel(type='Haser', molecule='H2O')        # doctest: +SKIP
+        >>> spec_model = SpectralModel(type='Haser', molecule='H2O')  # doctest: +SKIP
 
         >>> spec.fit(spec_model) # doctest: +SKIP
         >>> print(spec.fit_info) # doctest: +SKIP

sbpy/spectroscopy/tests/test_specgrad.py

@@ -37,6 +37,7 @@ def test_new_wave_error(self):
          0.21 * u.mag, 12 * u.percent / hundred_nm, 2),
         ((bandpass('SDSS g'), bandpass('SDSS r')),
          -0.15 * u.mag, -10 * u.percent / hundred_nm, 0.5),
+         ((500, 600) * u.nm, 3, 100 * u.percent / hundred_nm, 0.01),
     ))
     def test_from_color(self, wfb, color, S0, atol):
         """Test from color to spectral gradient.