ended double use of color to prevent misunderstanding

background is usually white in the browser
fill of the circle is default black

Black and white are also used in defining the mask, that's why I propose different colors for 'background' and the circle's fill

files/en-us/web/svg/element/mask/index.md

@@ -21,6 +21,7 @@ svg {
 
 ```html
 <svg viewBox="-10 -10 120 120">
+<rect x="-10" y="-10" width="120" height="120" fill="blue"/>
   <mask id="myMask">
     <!-- Everything under a white pixel will be visible -->
     <rect x="0" y="0" width="100" height="100" fill="white" />
@@ -34,7 +35,7 @@ svg {
   <polygon points="-10,110 110,110 110,-10" fill="orange" />
 
   <!-- with this mask applied, we "punch" a heart shape hole into the circle -->
-  <circle cx="50" cy="50" r="50" mask="url(#myMask)" />
+  <circle cx="50" cy="50" r="50" fill="purple" mask="url(#myMask)" />
 </svg>
 ```