Call function onPress React Native

In you're render you're setting up the handler incorrectly, give this a try;

 <View>
       <Icon
        name='heart'
        color={this.state.myColor}
        size= {45}
        style={{marginLeft:40}}
        onPress={this.handleClick}                                 
        />
  </View>

The syntax you're using would make sense for declaring an anonymous function inline or something but since your handler is defined on the class, you just reference it (not call it) using this.functionName in the props.

A little late to the party, but just wanted to leave this here if someone needs it

export default class mainScreen extends Component {

handleClick = () => {
 //some code
}

render() {
 return(
  <View>
       <Button
        name='someButton'
        onPress={() => {
            this.handleClick(); //usual call like vanilla javascript, but uses this operator
         }}                                 
        />
  </View>
  )};

You need to reference the method with the this keyword.

 <Icon
    onPress={this.handleClick}                                 
  />