Add multiple Sass tests

sass/utilities/functions.sass

@@ -62,6 +62,8 @@
   @return $value
 
 @function colorLuminance($color)
+  @if type-of($color) != 'color'
+    @return 0.55
   $color-rgb: ('red': red($color),'green': green($color),'blue': blue($color))
   @each $name, $value in $color-rgb
     $adjusted: 0
@@ -98,6 +100,11 @@
   @return $text-strong
 
 @function bulmaRgba($color, $alpha)
-  @if type-of($color) == 'color'
-    @return rgba($color, $alpha)
-  @return $color
+  @if type-of($color) != 'color'
+    @return $color
+  @return rgba($color, $alpha)
+
+@function bulmaDarken($color, $amount)
+  @if type-of($color) != 'color'
+    @return $color
+  @return darken($color, $amount)