you should not write this code as this is mutating the props object that react use.

Card.props = {color : 'blue'}

for proper usage the props are passed where you would be using the Card component

<Card color=  {"blue"} />

You can set the default props

const Card = (props) => {
    return (
        <div>
           <p style={{color: props.color}}>Random text</p> 
        </div>
    )
}

Card.defaultProps = {
    color: 'blue'
}


export default Card

You can assign your default prop values to the defaultProps property or even better take advantage of the default function parameters since your component is a function.

const Card = ({ color = 'blue' }) => {
  return (
    <div>
      <p style={{ color }}>Random text</p> 
    </div>
  )
}

export default Card;