Fieldtype.js

const Fieldtype = {
  mixins: [window.Storyblok.plugin],
  template: `<div class="uk-flex">
  <input class="uk-margin-right" v-model="model.color" type="color" />
  <input class="uk-width-1-1" v-model="model.color" /></div>`,
  methods: {
    initWith() {
      return {
        plugin: 'example_plugin',
        color: '#f40000'
      }
    },
    pluginCreated() {
      console.log('plugin:created')
    }
  },
  watch: {
    'model': {
      handler: function (value) {
        this.$emit('changed-model', value);
      },
      deep: true
    }
  }
}

Another possibility was introduced by klaus: https://github.com/klausfischer/storyblok-colorpicker/blob/master/index.js