Its actually a function of the default asp:RequiredFieldValidator - so below the dnn core and in the asp.net core
-  its default state of being is to display required field validators with a red font.

As such IF you place a asp:requiredFieldValidator on a form and DONT physically tell it to set ForeColor to no color it will assume red is the default and inject style="color:red;" into every form validator which overrides and css definitions in your skin by default.

The work arounds if its your own custom module or form are to either set ForeColor="" on the form aspx/ascx  or myValidator.ForeColor = Color.Empty; in your form code behind for any relevant validators.

In the dnn case since they have NOT put the ForeColor settings manually into validators - you canu simply override the defaults with css  -  instead you need to override with css !important. 

Westa

One workaround that we have used is to extend the asp:validators into your own controls and physically reset the default if nothing is explicitly define - but it is kinda one of those dumb - what were they thinking things in ASP.NET