Fix/use "new" to construct a new instance of Color (#227)

* Add new

* Add tests

* Add tests

src/simularium/VisGeometry/color-utils.ts

@@ -11,5 +11,5 @@ export function convertColorNumberToString(color: number | string): string {
     if (typeof color === "string") {
         return color;
     }
-    return "#" + Color(color).getHexString();
+    return "#" + new Color(color).getHexString();
 }

src/test/color-utils.test.ts

@@ -0,0 +1,23 @@
+import {
+    convertColorStringToNumber,
+    convertColorNumberToString,
+} from "../simularium/VisGeometry/color-utils";
+
+describe("VisGeometry color-utils", () => {
+    describe("convertColorStringToNumber", () => {
+        test("returns a color number as is", () => {
+            expect(convertColorStringToNumber(16777215)).toEqual(16777215);
+        });
+        test("converts a hex color string to a base-16 number", () => {
+            expect(convertColorStringToNumber("#ffffff")).toEqual(16777215);
+        });
+    });
+    describe("convertColorNumberToString", () => {
+        test("returns a color string as is", () => {
+            expect(convertColorNumberToString("#ffffff")).toEqual("#ffffff");
+        });
+        test("converts a base-16 color number to a hex color string", () => {
+            expect(convertColorNumberToString(16777215)).toEqual("#ffffff");
+        });
+    });
+});