Tap on the tool that you are using to change it's size and opacity.

Use the colour picker to change the colour opacity for adding shadow and highlights.

Use the pencil tool for adding colour and the pen tool for adding detail.

Tap on the preferences icon in the top right corner for more options.

Show the original image behind your artwork and use the colour picker to get the correct colour.

Draw and colour with your fingertips or use the Apple Pencil for greater precision.