Finally I get on moodle.css (* Bootstrap v2.3.0
*
* Copyright 2012 Twitter, Inc
* Licensed under the Apache License v2.0
* http://www.apache.org/licenses/LICENSE-2.0
)
that the red color was effectively defined as .que .validationerror{color:#b94a48} that I modify to #f94aff wich give the following
So a proposal to only color the text in the input field for numerical could be the one to use as it respects the validation error settings and give the user where to go. As he will get close to the input field, the pop-up will be activated.