To change colors, tap on any of the brushes in your toolset. In the menu that opens, tap on the Color circle and the color wheel (and other options for changing the color) will open. Once you've chosen one, tap the + icon to save it to your brush.
This tutorial might come in handy, it's a brief walkthrough of most of Draw's basic functions: Use Adobe Illustrator Draw to craft scalable vector designs that can be edited in Illustrator CC and placed in Photoshop…
Let me know, though, if you have other questions.
If you want to change the color of a shape you have already drawn you can do that by tapping and holding down on the shape. Give it a moment or two and the color will change to the color of the selected brush tool. Make sure you have selected a brush with the color you want to use first. I know this wasn't obvious to me when I first started using Draw.
You always come to my rescue. I COMPLETELY misread/misunderstood his query.