I find the solution.
It was a css conflict.
In my case in the file styles.css of your plugin, search the span.wpcf7-not-valid-tip tag and add color (color :red)
span.wpcf7-not-valid-tip {
position: absolute;
top: 20%;
left: 20%;
z-index: 100;
background: #fff;
border: 1px solid #ff0000;
font-size: 10pt;
width: 280px;
padding: 2px;
color:red; // Here is the color
}
It must work.