How to add colored border on cardview?

As the accepted answer requires you to add a Frame Layout, here how you can do it with material design.

Add this if you haven't already

implementation 'com.google.android.material:material:1.0.0'

Now change to Cardview to MaterialCardView

<com.google.android.material.card.MaterialCardView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="16dp"
app:cardCornerRadius="8dp"
app:cardElevation="2dp"

app:strokeWidth="1dp"
app:strokeColor="@color/black">

Now you need to change the activity theme to Theme.Material. If you are using Theme.Appcompact I will suggest you to move to Theme.Material for future projects for having better material design in you app.

    <style name="AppTheme" parent="Theme.MaterialComponents.DayNight">

Start From the material design update, it support app:strokeColor and also app:strokeWidth. see more

to use material design update. add following code to build.gradle(:app)

dependencies {
    // ...
    implementation 'com.google.android.material:material:1.0.0'
    // ...
  }

and Change CardView to MaterialCardView

try doing:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    xmlns:card_view="http://schemas.android.com/apk/res-auto"
    card_view:cardElevation="2dp"
    card_view:cardCornerRadius="5dp">

    <FrameLayout
        android:background="#FF0000"
        android:layout_width="4dp"
        android:layout_height="match_parent"/>

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:padding="16dp"
        android:orientation="vertical">

        <TextView
            style="@style/Base.TextAppearance.AppCompat.Headline"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="Title" />

        <TextView
            style="@style/Base.TextAppearance.AppCompat.Body1"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="Content here" />

    </LinearLayout>

</android.support.v7.widget.CardView>

this removes the padding from the cardview and adds a FrameLayout with a color. You then need to fix the padding in the LinearLayout then for the other fields

Update

If you want to preserve the card corner radius create card_edge.xml in drawable folder:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >
    <solid android:color="#F00" />
    <size android:width="10dp"/>
    <padding android:bottom="0dp" android:left="0dp" android:right="0dp" android:top="0dp"/>
    <corners android:topLeftRadius="5dp" android:bottomLeftRadius="5dp"
        android:topRightRadius="0.1dp" android:bottomRightRadius="0.1dp"/>
</shape>

and in the frame layout use android:background="@drawable/card_edge"