chromium/overlay/frameworks/base/core/res/res/drawable/webviewchromium_color_button_background.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
     Copyright 2013 The Chromium Authors. All rights reserved.
     Use of this source code is governed by a BSD-style license that can be
     found in the LICENSE file.
-->

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:top="0dp"
        android:right="0dp"
        android:bottom="0dp"
        android:left="0dp">
        <shape android:shape="rectangle">
            <solid android:color="@color/webviewchromium_color_picker_border_color"/>
        </shape>
    </item>
    <item android:top="0px"
        android:right="1px"
        android:bottom="0px"
        android:left="1px">
        <shape android:shape="rectangle">
            <solid android:color="@color/webviewchromium_color_picker_background_color"/>
        </shape>
    </item>
    <item
        android:id="@+id/webviewchromium_color_button_swatch"
        android:top="1px"
        android:right="2px"
        android:bottom="1px"
        android:left="2px">
        <shape android:shape="rectangle">
            <solid android:color="#FF0000"/>
        </shape>
    </item>
</layer-list>