diff --git a/rasterio/_io.pyx b/rasterio/_io.pyx
index f7291165..3ce31a60 100644
--- a/rasterio/_io.pyx
+++ b/rasterio/_io.pyx
@@ -1680,14 +1680,26 @@ cdef class RasterUpdater(RasterReader):
         # GPI_Gray=0,  GPI_RGB=1, GPI_CMYK=2,     GPI_HLS=3
         hTable = _gdal.GDALCreateColorTable(1)
         vals = range(256)
+
         for i, rgba in colormap.items():
+
+            if len(rgba) == 4 and self.driver in ('GTiff'):
+                raise ValueError(
+                    "Format '%s' doesn't support 4 component colormap entries"
+                    % self.driver)
+
+            elif len(rgba) == 3:
+                rgba = tuple(rgba) + (255,)
+
             if i not in vals:
                 log.warn("Invalid colormap key %d", i)
                 continue
+
             color.c1, color.c2, color.c3, color.c4 = rgba
             _gdal.GDALSetColorEntry(hTable, i, &color)
+
         # TODO: other color interpretations?
-        _gdal.GDALSetRasterColorInterpretation(hBand, 2)
+        _gdal.GDALSetRasterColorInterpretation(hBand, 1)
         _gdal.GDALSetRasterColorTable(hBand, hTable)
         _gdal.GDALDestroyColorTable(hTable)
 
@@ -1696,9 +1708,9 @@ cdef class RasterUpdater(RasterReader):
         mask.
 
         The optional `window` argument takes a tuple like:
-        
+
             ((row_start, row_stop), (col_start, col_stop))
-            
+
         specifying a raster subset to write into.
         """
         cdef void *hband
diff --git a/tests/test_colormap.py b/tests/test_colormap.py
index e5899fba..112ebfad 100644
--- a/tests/test_colormap.py
+++ b/tests/test_colormap.py
@@ -7,6 +7,21 @@ import rasterio
 
 logging.basicConfig(stream=sys.stderr, level=logging.DEBUG)
 
+
+def test_write_colormap_err(tmpdir):
+
+    with rasterio.drivers():
+
+        with rasterio.open('tests/data/shade.tif') as src:
+            meta = src.meta
+
+        tiffname = str(tmpdir.join('foo.tif'))
+
+        with rasterio.open(tiffname, 'w', **meta) as dst:
+            with pytest.raises(ValueError):
+                dst.write_colormap(1, {0: (255, 0, 0, 255), 255: (0, 0, 0, 0)})
+
+
 def test_write_colormap(tmpdir):
 
     with rasterio.drivers():
@@ -15,19 +30,20 @@ def test_write_colormap(tmpdir):
             shade = src.read_band(1)
             meta = src.meta
 
-        tiffname = str(tmpdir.join('foo.tif'))
-        
+        tiffname = str(tmpdir.join('foo.png'))
+        meta['driver'] = 'PNG'
+
         with rasterio.open(tiffname, 'w', **meta) as dst:
             dst.write_band(1, shade)
-            dst.write_colormap(1, {0: (255, 0, 0, 255), 255: (0, 0, 255, 255)})
+            dst.write_colormap(1, {0: (255, 0, 0, 255), 255: (0, 0, 0, 0)})
             cmap = dst.colormap(1)
             assert cmap[0] == (255, 0, 0, 255)
-            assert cmap[255] == (0, 0, 255, 255)
+            assert cmap[255] == (0, 0, 0, 0)
 
         with rasterio.open(tiffname) as src:
             cmap = src.colormap(1)
             assert cmap[0] == (255, 0, 0, 255)
-            assert cmap[255] == (0, 0, 255, 255)
+            assert cmap[255] == (0, 0, 0, 0)
 
     # subprocess.call(['open', tiffname])