Change text input color for hashtags react native mobx

I want to dynamically change the color of words in a textInput element where a hashtag comes up. Similar to a lot of apps. Here is what i have so far:

@observable descriptionString: any = '';
@observable formattedText: any = '';

handleCheckTextChange = (inputText:any) => {
    const words = inputText.split(' ');
    console.log(words);
    const formattedText:any = [];
    words.forEach((word:any, index:any) => {
      const isLastWord = index === words.length - 1;
      if (!word.startsWith('@')) {
        return isLastWord ? formattedText.push(word) : formattedText.push(word, ' ');
      }
      const mention = (
        <Text key={word + index} style={{color: 'red'}}>
          {word}
        </Text>
      );
      isLastWord ? formattedText.push(mention) : formattedText.push(mention, ' ');
    });
    this.formattedText = formattedText;
  }

The text input:

<TextInput
            maxLength={descriptionMaxLength}
            multiline={true}
            placeholder='Type description here'
            style={styles.descriptionInput}
            returnKeyType='next'
            blurOnSubmit={true}
            onChangeText={(text) => this.handleCheckTextChange(text)}
          >
            <Text>{this.formattedText}</Text>
          </TextInput>

I got this from another stack overflow question, but it doesn't seem to work with my code. Getting a mobx error saying "Dynamic observable objects cannot be frozen." I am using mobx for state handling and the error is a mobx error so i am think that this may be an issue with setting and changing that formattedText observable. Any help is appreciated!