Fix thresholds default colors not being applied

packages/grafana-ui/src/components/ColorPicker/_ColorPicker.scss

@@ -199,6 +199,7 @@ $arrowSize: 15px;
   margin: 0;
   float: left;
   z-index: 0;
+  background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAwAAAAMCAIAAADZF8uwAAAAGUlEQVQYV2M4gwH+YwCGIasIUwhT25BVBADtzYNYrHvv4gAAAABJRU5ErkJggg==);
 }
 
 .sp-preview-inner,

public/app/plugins/panel/graph/threshold_manager.ts

@@ -198,6 +198,7 @@ export class ThresholdManager {
       }
 
       let fillColor, lineColor;
+
       switch (threshold.colorMode) {
         case 'critical': {
           fillColor = 'rgba(234, 112, 112, 0.12)';
@@ -215,6 +216,12 @@ export class ThresholdManager {
           break;
         }
         case 'custom': {
+          if (!threshold.fillColor) {
+            threshold.fillColor = 'rgba(255, 255, 255, 1)';
+          }
+          if (!threshold.lineColor) {
+            threshold.lineColor = 'rgba(255, 255, 255, 0)';
+          }
           fillColor = threshold.fillColor;
           lineColor = threshold.lineColor;
           break;