To paint a portrait, Maxwell will create something to paint with and something to paint on. Use the following answers:

Creating something to paint on:
Round 1: Canvas
Round 2: Paper
Round 3: Wall

Paint with the following tools:
Round 1: Brush (Paint)
Round 2: Pencil
Round 3: Pen

Scribblenauts Remix 10-6 Cheats Hints gallery:
10-6-1-a: Paint a portrait!
10-6-1-b: Maxwell will need some paint.
10-6-1-c: Apply the paint to a canvas.