do not reset color-by when subsetting world (#636)

* do not reset colors when subsetting to world

* revert diffexp state change

client/src/components/graph/graph.js

@@ -30,7 +30,7 @@ import { World } from "../../util/stateManager";
   universe: state.controls.universe,
   crossfilter: state.controls.crossfilter,
   responsive: state.responsive,
-  colorRGB: _.get(state.controls, "colorRGB", null),
+  colorRGB: _.get(state.controls, "colors.rgb", null),
   opacityForDeselectedCells: state.controls.opacityForDeselectedCells,
   selectionUpdate: _.get(state.controls, "crossfilter.updateTime", null),
   resettingInterface: state.controls.resettingInterface,
@@ -159,11 +159,7 @@ class Graph extends React.Component {
       }
 
       // Colors for each point - a cached value that only changes when
-      // the cell metadata changes (done by updateCellColors middleware).
-      // NOTE: this is a slightly pessimistic assumption, as the metadata
-      // could have changed for some other reason, but for now color is
-      // the only metadata that changes client-side.  If this is problematic,
-      // we could add some sort of color-specific indicator to the app state.
+      // the cell metadata changes.
       if (!renderCache.colors || colorRGB !== prevProps.colorRGB) {
         const rgb = colorRGB;
         if (!renderCache.colors) {