Parcel gives you the ability to simulate a variety of visual impairments to help you design inclusive emails that can be read and understood by all. You can enable/disable any of these by accessing the Visual Impairment menu in the upper right corner of the Preview. You can only have one filter applied at a time.
As an example, let's see how a colorful email looks with each filter.
For more information about inclusive accessible design in email, please see some of these resources.