Blue highlights stands for style.
When you are addressing unknown people, the use of “tu” is consider informal and as such, potentially disrespectful. You may not have dealt with it yet, since it depends on the environment that you move in, and which people you usually address but this distinction is not unique to European Portuguese.
False positive is when a rule detects something that it is not meant to detect, but this rule was designed to detect this and it was actually designed to have only false negatives, which it does in a great amount.
Error types are designed independently, so, if this rule does not apply to your writing style, just click “Ignore this type of error”.
I was trying to create a rule but it gives errors in TESTRULES PT.
What is wrong with this?:
<rule id='PREFERIR_ANTES' name="Preferir">
<!-- Created by Marco A.G.Pinto, Portuguese rule -->
<message>Substitua por <suggestion><match no='1' postag='V.+' postag_regexp='yes'>preferir</match></suggestion>.</message>
<example correction="prefere">A Ana <marker>prefere antes</marker> a Matemática.</example>