Its a little confusing but tints are just fills on a Group
or a SymbolInstance
So:
let sketch = require('sketch')
let doc = sketch.getSelectedDocument()
let selection = doc.selectedLayers.layers[0]
console.log(selection.type)
// 'SymbolInstance'
console.log(selection.style.fills)
// [Fill { fillType: 'Color', color: '#ff0000ff’…}]
https://developer.sketch.com/plugins/updates/new-in-sketch-64