Change Circle color of radio button

More simple, just set the buttonTint color: (only works on api level 21 or above)

<RadioButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/radio"
    android:checked="true"
    android:buttonTint="@color/your_color"/>

in your values/colors.xml put your color in this case a reddish one:

<color name="your_color">#e75748</color>

Result:

If you want to do it by code (also api 21 and above):

if(Build.VERSION.SDK_INT>=21)
{

    ColorStateList colorStateList = new ColorStateList(
            new int[][]{

                    new int[]{-android.R.attr.state_enabled}, //disabled
                    new int[]{android.R.attr.state_enabled} //enabled
            },
            new int[] {

                    Color.BLACK //disabled
                    ,Color.BLUE //enabled

            }
        );                       


    radio.setButtonTintList(colorStateList);//set the color tint list
    radio.invalidate(); //could not be necessary
}

Update: 1. use this one instead

   <android.support.v7.widget.AppCompatRadioButton
        android:id="@+id/rbtn_test"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:buttonTint="@color/primary" />

2. Then add this line into parent layout or Alt + Enter in Android Studio to auto-add xmlns:app="http://schemas.android.com/apk/res-auto"

Minimum Example should look like this:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <android.support.v7.widget.AppCompatRadioButton
        android:id="@+id/rbtn_test"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:buttonTint="@color/primary" />

</LinearLayout>

3. In your program, should call like this. AppCompatRadioButton radioButton = (AppCompatRadioButton) view.findViewById(R.id.rbtn_test);

Basically, this kind of pattern can be applied for all AppCompact types such as AppCompatCheckBox, AppCompatButton and so on.

Old Answer:

In order to support below android API 21, you can use AppCompatRadioButton. Then use setSupportButtonTintList method to change the color. This is my code snippet to create a radio button .

    AppCompatRadioButton rb;
    rb = new AppCompatRadioButton(mContext);

    ColorStateList colorStateList = new ColorStateList(
            new int[][]{
                    new int[]{-android.R.attr.state_checked},
                    new int[]{android.R.attr.state_checked}
            },
            new int[]{

                    Color.DKGRAY
                    , Color.rgb (242,81,112),
            }
    );
    rb.setSupportButtonTintList(colorStateList);

Tested result at API 19:

See the android reference link for more detail.

<android.support.v7.widget.AppCompatRadioButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:buttonTint="@color/Color" />