The text color in the image editor can be altered from your vBulletin admin panel->Styles & Templates->Style Manager->Edit Templates->Go->CSS Templates->editor.css.
Search for the .editoroverlay.imageoverlay div.imagetarget definition and enter the preferred code in it using the following line:
color: COLOR;
Replace COLOR with the color name or the corresponding HEX code.
The definition will be similar to the following one:
.editoroverlay.imageoverlay div.imagetarget {
margin-{vb:stylevar left}:25%;
border:solid 1px {vb:stylevar mid_border.borderColor};
padding:5px;
color: green;
font-size:{vb:stylevar small_fontSize};
background-color:{vb:stylevar content3_background.backgroundColor};;
}