A Polygon2D has a list of vertices. You can use that list in combination with beginShape-endShape and the right renderer to create gradients.

Note that only the P2D, P3D and OPENGL renderers facilitate per vertex color changes (aka gradients) within a beginShape-endShape. With P2D and P3D there may appear white-ish lines in your triangulated shape. With OPENGL there may be jagged edges.

Code Example

1. // LEFT mouse button adds points, RIGHT mouse button finalizes shape
2. // 'd' deletes polygon, SPACE clears everything
3. 
   
4. import toxi.geom.*;
5. ArrayList <Polygon2D> polygons = new ArrayList <Polygon2D> ();
6. ArrayList <Vec2D> points = new ArrayList <Vec2D> ();
7. Vec2D mouse;
8. 
   
9. void setup() {
10.   size(1280,720,P2D);
11.   noStroke();
12.   smooth();
13.   colorMode(HSB,360,100,100,100);
14. }
15. 
    
16. void draw() {
17.   background(180,0,100,100);
18.   mouse = new Vec2D(mouseX,mouseY);
19. 
    
20.   // draw all the polygons
21.   for (Polygon2D p : polygons) {
22.     int numVertices = p.vertices.size();
23.     beginShape();
24.     for (int i=0; i<numVertices; i++) {
25.       Vec2D v = p.vertices.get(i);
26.       float c = map(i,0,numVertices,0,360);
27.       fill(c,100,100,50);
28.       vertex(v.x,v.y);
29.     }
30.     endShape(CLOSE);
31.   }
32. 
    
33.   // preview shape
34.   int numPoints = points.size();
35.   beginShape();
36.   for (int i=0; i<numPoints; i++) {
37.     Vec2D v = points.get(i);
38.     float c = map(i,0,numPoints,0,360);
39.     fill(c,100,100,50);
40.     vertex(v.x,v.y);
41.   }
42.   endShape(CLOSE);
43. 
    
44.   // draw all the points
45.   fill(0);
46.   for (Vec2D p : points) {
47.     ellipse(p.x,p.y,5,5);
48.   }
49. }
50. 
    
51. void mousePressed() {
52.   if (mouseButton == LEFT) {
53.     points.add(mouse);
54.   } else if (mouseButton == RIGHT && points.size() > 2) {
55.     // create a polygon from the points
56.     polygons.add(new Polygon2D(points));
57.     // clear the points
58.     points.clear();
59.   }
60. }
61. 
    
62. void keyPressed() {
63.   // clear all points and polygons
64.   if (key == ' ' && !mousePressed) {
65.     points.clear();
66.     polygons.clear();
67.   }
68.   // delete the polygon under the mouse
69.   if (key == 'd' && !mousePressed) {
70.     for (int i=polygons.size()-1; i>=0; i--) {
71.       if (polygons.get(i).containsPoint(mouse)) {
72.         polygons.remove(i);
73.       }
74.     }
75.   }
76. }