# How to change the colors of objects in VTK when using vtkAppendPolyData?
**URL:** https://discourse.vtk.org/t/how-to-change-the-colors-of-objects-in-vtk-when-using-vtkappendpolydata/2431
**Category:** Support
**Created:** [January 15, 2020, 2:26pm UTC](https://discourse.vtk.org/t/how-to-change-the-colors-of-objects-in-vtk-when-using-vtkappendpolydata/2431 "2020-01-15T14:26:18Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@ymmx](https://discourse.vtk.org/u/ymmx)
#### Post date: [January 15, 2020, 2:26pm UTC](https://discourse.vtk.org/t/how-to-change-the-colors-of-objects-in-vtk-when-using-vtkappendpolydata/2431/1 "2020-01-15T14:26:18Z")
I’m trying to modify the color of each object that I’ve placed in a `vtkAppendPolyData` but I get weird colors. I mean the object is not in one color but each face of it has a different color.
I would like to have one color by object.
Here is the minimal example:
```
import vtk
sphereSource = vtk.vtkSphereSource()
sphereSource.SetCenter(5,0,0)
sphereSource.Update()
Colors = vtk.vtkUnsignedCharArray()
Colors.SetNumberOfComponents(3)
Cellarray = sphereSource.GetOutput().GetPolys().GetNumberOfCells()
Colors.SetNumberOfTuples(Cellarray)
for c in range(Cellarray):
Colors.InsertNextTuple3(255, 0, 0)
sphereSource.GetOutput().GetCellData().SetScalars(Colors)
sphereSource.Update()
coneSource =vtk.vtkConeSource()
coneSource.Update()
Colors = vtk.vtkUnsignedCharArray()
Colors.SetNumberOfComponents(3)
Cellarray = coneSource.GetOutput().GetPolys().GetNumberOfCells()
Colors.SetNumberOfTuples(Cellarray)
for c in range(Cellarray):
Colors.InsertNextTuple3(0, 255, 0)
coneSource.GetOutput().GetCellData().SetScalars(Colors)
coneSource.Update()
# Append the two meshes
appendFilter = vtk.vtkAppendPolyData()
appendFilter.AddInputData(sphereSource.GetOutput())
appendFilter.AddInputData(coneSource.GetOutput())
appendFilter.Update()
# Remove any duplicate points.
cleanFilter = vtk.vtkCleanPolyData()
cleanFilter.SetInputConnection(appendFilter.GetOutputPort())
cleanFilter.Update()
# Create a mapper and actor
mapper = vtk.vtkPolyDataMapper()
# mapper.ScalarVisibilityOn()
mapper.SetInputConnection(cleanFilter.GetOutputPort())
mapper.SetColorModeToDirectScalars()
actor = vtk.vtkActor()
actor.SetMapper(mapper)
# Create a renderer, render window, and interactor
renderer = vtk.vtkRenderer()
renderWindow = vtk.vtkRenderWindow()
renderWindow.AddRenderer(renderer)
renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.SetRenderWindow(renderWindow)
# Add the actors to the scene
renderer.AddActor(actor)
renderer.SetBackground(.3, .2, .1) # Background color dark red
# Render and interact
renderWindow.Render()
renderWindowInteractor.Start()
```
---
### Author:  [@normanius](https://discourse.vtk.org/u/normanius)
#### Post date: [January 15, 2020, 11:10pm UTC](https://discourse.vtk.org/t/how-to-change-the-colors-of-objects-in-vtk-when-using-vtkappendpolydata/2431/2 "2020-01-15T23:10:38Z")
You handle the data arrays `colors` wrongly. You first allocate `nTuples` tuples (`colors.SetNumberOfTuples()`) and then insert new elements at the end of that array (`colors.InsertNextTuple()`). This way, you end up with an array of size `2*nTuples`. The first half of tuples are not initialized (that’s why you see the funky color patterns), and the second half of tuples actually are red or green.
The solution: Either use `InsertNextTuple(values)` or `SetNumberOfTuples(n)` in combination with `SetTuple(i, values)`, but never mix these two approaches.
The code:
```
import vtk
# Sphere in red.
sphereSource = vtk.vtkSphereSource()
sphereSource.SetCenter(5,0,0)
sphereSource.Update()
sphereSource = sphereSource.GetOutput() # vtkPolyData()
colors = vtk.vtkUnsignedCharArray()
colors.SetNumberOfComponents(3)
colors.SetNumberOfTuples(sphereSource.GetNumberOfCells())
for c in range(sphereSource.GetNumberOfCells()):
colors.SetTuple(c, [255, 0, 0])
sphereSource.GetCellData().SetScalars(colors)
# Cone in green.
coneSource =vtk.vtkConeSource()
coneSource.Update()
coneSource = coneSource.GetOutput() # vtkPolyData()
colors = vtk.vtkUnsignedCharArray()
colors.SetNumberOfComponents(3)
colors.SetNumberOfTuples(coneSource.GetNumberOfCells())
for c in range(coneSource.GetNumberOfCells()):
colors.SetTuple(c, [0, 255, 0])
coneSource.GetCellData().SetScalars(colors)
# Combine the two meshes
appendFilter = vtk.vtkAppendPolyData()
appendFilter.AddInputData(sphereSource)
appendFilter.AddInputData(coneSource)
appendFilter.Update()
# Create a mapper and actor
mapper = vtk.vtkPolyDataMapper()
mapper.SetInputConnection(appendFilter.GetOutputPort())
mapper.SetColorModeToDirectScalars()
actor = vtk.vtkActor()
actor.SetMapper(mapper)
# Create a renderer, render window, and interactor
renderer = vtk.vtkRenderer()
renderWindow = vtk.vtkRenderWindow()
renderWindow.AddRenderer(renderer)
renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.SetRenderWindow(renderWindow)
# Add the actors to the scene
renderer.AddActor(actor)
renderer.SetBackground(.3, .2, .1)
# Render and interact
renderWindow.Render()
renderWindowInteractor.Start()
```
---
### Author:  [@ymmx](https://discourse.vtk.org/u/ymmx)
#### Post date: [January 16, 2020, 8:28am UTC](https://discourse.vtk.org/t/how-to-change-the-colors-of-objects-in-vtk-when-using-vtkappendpolydata/2431/3 "2020-01-16T08:28:17Z")
Okay. I get it.
I thought `SetNumberOfTuples` allocates the number of tuples I would like to have but I was wrong.
Thanks you so much !
---
### Author:  [@normanius](https://discourse.vtk.org/u/normanius)
#### Post date: [January 16, 2020, 10:29am UTC](https://discourse.vtk.org/t/how-to-change-the-colors-of-objects-in-vtk-when-using-vtkappendpolydata/2431/4 "2020-01-16T10:29:53Z")
`SetNumberOfTuples` does allocate the number tuples you want. But `InsertNextTuple` increases that number again. It’s fine to use either of those, but better don’t use them together.