What should be the color of the Ripple, colorPrimary or colorAccent? (Material Design)

Use 26% alpha for colored ripples.

Android L doesn't support theme attributes in color state lists and the <ripple> element doesn't have an alpha channel, but for bounded ripples you can do something like:

<ripple xmlns:android="http://schemas.android.com/apk/res/android"        android:color="?android:attr/colorAccent">    <item android:id="@android:id/mask">        <color android:color="#42ffffff" />    </item></ripple>

This won't work for an unbounded ripple. Alternatively, since you know your own accent color, you can either define a 26% alpha version or use a color state list. Either of these will work fine for an unbounded ripple.

<ripple xmlns:android="http://schemas.android.com/apk/res/android"        android:color="@color/accent_26" />

res/values/colors.xml:

<resources>    ...    <color name="accent">#ff33b5e5</color>    <color name="accent_alpha26">#4233b5e5</color></resources>

res/color/accent_alpha26.xml:

<selector xmlns:android="http://schemas.android.com/apk/res/android">    <item android:color="@color/accent"          android:alpha="0.26" /></selector>

Here is what you are looking for:

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">   ...   <item name="colorControlHighlight">@color/ripple_material_dark</item>   ...</style>

The Widget.Material.Button style uses the btn_default_material.xml as its background:

<ripple xmlns:android="http://schemas.android.com/apk/res/android"    android:color="?attr/colorControlHighlight">  <item android:drawable="@drawable/btn_default_mtrl_shape" /></ripple>

Note that the ripple color is ?attr/colorControlHighlight which by default is #40ffffff or #40000000 (for dark and light themes, respectively) as mentioned in the touch feedback guide.

As you've noticed, ripples are used subtle indications of touch feedback, hence why they do not use colorPrimary or colorAccent by default. This is consistent with the changes made in Android 4.4 (Kitkat) which made the default selector colors neutral by default.