Document.Paint(ElementId, Face, ElementId)

public void ApplyPaintByMaterial(Document document, Wall wall, Material material)
{
   // Before acquiring the geometry, make sure the detail level is set to 'Fine'
   Options geoOptions = new Options();
   geoOptions.DetailLevel = ViewDetailLevel.Fine;

   // Obtain geometry for the given Wall element
   GeometryElement geoElem = wall.get_Geometry(geoOptions);

   // Find a face on the wall
   Face wallFace = null;
   IEnumerator<GeometryObject> geoObjectItor = geoElem.GetEnumerator();
   while (geoObjectItor.MoveNext())
   {
      // need to find a solid first
      Solid theSolid = geoObjectItor.Current as Solid;
      if (null != theSolid)
      {
         // Examine faces of the solid to find one with at least
         // one region. Then take the geometric face of that region.
         foreach (Face face in theSolid.Faces)
         {
            if (face.HasRegions)
            {
               wallFace = face.GetRegions()[0];
               break;
            }
         }
      }
   }

   if (null == wallFace)
   {
      TaskDialog.Show("Failure", "Could not find a face to paint on the given wall.");
      return;
   }

   // Paint material to the wall face (modification must be inside a transaction)

   using (Transaction transaction = new Transaction(document, "Painting a wall"))
   {
      transaction.Start();
      document.Paint(wall.Id, wallFace, material.Id);
      transaction.Commit();
   }

   // For illustration purposes only, check if the painted material indeed got applied

   bool isPainted = document.IsPainted(wall.Id, wallFace);
   if (isPainted)
   {
      ElementId paintedMatId = document.GetPaintedMaterial(wall.Id, wallFace);
      if (paintedMatId == material.Id)
      {
         TaskDialog.Show("Painting material", "Wall painted successfully.");
      }
   }
}

' Paint any unpainted faces of a given wall
Public Sub PaintWallFaces(wall As Wall, matId As ElementId)
   Dim doc As Document = wall.Document
   Dim geometryElement As GeometryElement = wall.Geometry(New Options())
   For Each geometryObject As GeometryObject In geometryElement
      If TypeOf geometryObject Is Solid Then
         Dim solid As Solid = TryCast(geometryObject, Solid)
         For Each face As Face In solid.Faces
            If doc.IsPainted(wall.Id, face) = False Then
               doc.Paint(wall.Id, face, matId)
            End If
         Next
      End If
   Next
End Sub

Public Sub ApplyPaintByMaterial(document As Document, wall As Wall, material As Material)
    ' Before acquiring the geometry, make sure the detail level is set to 'Fine'
    Dim geoOptions As New Options()
    geoOptions.DetailLevel = ViewDetailLevel.Fine

    ' Obtain geometry for the given Wall element
    Dim geoElem As GeometryElement = wall.Geometry(geoOptions)

    ' Find a face on the wall
    Dim wallFace As Face = Nothing
    Dim geoObjectItor As IEnumerator(Of GeometryObject) = geoElem.GetEnumerator()
    While geoObjectItor.MoveNext()
        ' need to find a solid first
        Dim theSolid As Solid = TryCast(geoObjectItor.Current, Solid)
        If theSolid IsNot Nothing Then
            ' Examine faces of the solid to find one with at least
            ' one region. Then take the geometric face of that region.
            For Each face As Face In theSolid.Faces
                If face.HasRegions Then
                    wallFace = face.GetRegions()(0)
                    Exit For
                End If
            Next
        End If
    End While

    If wallFace Is Nothing Then
        TaskDialog.Show("Failure", "Could not find a face to paint on the given wall.")
        Return
    End If

    ' Paint material to the wall face (modification must be inside a transaction)

    Using transaction As New Transaction(document, "Painting a wall")
        transaction.Start()
        document.Paint(wall.Id, wallFace, material.Id)
        transaction.Commit()
    End Using

    ' For illustration purposes only, check if the painted material indeed got applied

    Dim isPainted As Boolean = document.IsPainted(wall.Id, wallFace)
    If isPainted Then
        Dim paintedMatId As ElementId = document.GetPaintedMaterial(wall.Id, wallFace)
        If paintedMatId = material.Id Then
            TaskDialog.Show("Painting material", "Wall painted successfully.")
        End If
    End If
End Sub