I used the grey-skin and added 2 inputs, one of type 'text' and the other of type 'number'.
When i focus on these inputs, the blue color of the bottom border are not the same.
Looking in the file mdb.css, i noticed that the default color is #4285f4 (border-bottom: 1px solid #4285f4;), but this color is overwritten with a more specific rule

.grey-skin input[type="email"]:focus:not([readonly]),
.grey-skin input[type="text"]:focus:not([readonly]),
.grey-skin input[type="password"]:focus:not([readonly]),
.grey-skin textarea.md-textarea:focus:not([readonly]) {
  border-color: #75c1f0;

So for inputs of type email|text|password, the border is #75C1F0, for the others is #4285F4.
I would expect all border have the same color :D

You can verify the issue with the following code (taken from your site)

<!-- Medium input -->
<div class="md-form">
<input type="text" id="inputMDEx1" class="form-control">
<label for="inputMDEx1">Medium input</label>
</div>
<!-- Medium input -->
<div class="md-form">
<input type="number" id="inputMDEx2" class="form-control">
<label for="inputMDEx2">Medium input</label>
</div>