Added a test specifically to cover the NRGBA interface handling, so that I could get 100% coverage.

oklab.go

@@ -194,5 +194,8 @@ var (
 	Model = color.ModelFunc(okLabModel)
 
 	// Type assertions.
-	_ NRGBAColor = Color{}
+	_ interface {
+		color.Color
+		NRGBAColor
+	} = Color{}
 )

oklab_test.go

@@ -76,6 +76,18 @@ func fixedNRGBA64Model(c color.Color) color.Color {
 	return color.NRGBA64Model.Convert(c)
 }
 
+type testNRGBAColor struct {
+	color.NRGBA64
+}
+
+func (c testNRGBAColor) NRGBA() (r, g, b, a uint32) {
+	return uint32(c.R), uint32(c.G), uint32(c.B), uint32(c.A)
+}
+
+func testNRGBAModel(c color.Color) color.Color {
+	return testNRGBAColor{fixedNRGBA64Model(c).(color.NRGBA64)}
+}
+
 func Test_Model(t *testing.T) {
 	// test to make sure we can reproduce some test colors.
 	// In particular, I want to make sure the colors can be recovered from transparent NRGBA and NRGBA64.
@@ -136,6 +148,13 @@ func Test_Model(t *testing.T) {
 			"RGBA: invisible nothing",
 			color.RGBA{R: 0x0, G: 0x0, B: 0x0, A: 0x0},
 			color.RGBAModel,
+		}, {
+			// test the handling of the NRGBAColor interface; as
+			// all the types that support it have special handling
+			// that would take precidence.
+			"testNRGBAColor: invisible Floral White",
+			testNRGBAColor{color.NRGBA64{R: 0xff, G: 0xfa, B: 0xf0, A: 0x00}},
+			color.ModelFunc(testNRGBAModel),
 		},
 	} {
 		t.Run(tt.name, func(t *testing.T) {