Q:

R is not resolved

Hey ive had the R not resolved problem for a while, so i started a new project and realized the the R not resolved problem is becouse i added a android:background="#09900" to a button in the XML :
 <Button
       android:id="@+id/btnMap"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:background="#09900"
       android:text="map"
       app:layout_constraintBottom_toTopOf="parent" />

When i remove the background, its all fine. So i still have another project which is quite a bit larger that has the R not resolved problem. but i cant find what is causing it because even here the background="#09900" is not registered as an error or warning. Any idea what i should look for or how i can find the problem more easily ?
EDIT: I realize the error in this code, i was hoping for ways or tips to avoid errors similar to this, since the editor does not warn you about them. I still have a large project with an error like this and i have no idea where to look for it. Im not asking about the error in the code provided here.

A:

Set color code in color file. It should be  6 or 8 digit code and not 5. 
'android:background="#09900"'

