Why does some faces look like this?

[Jep]

http://imgur.com/njUo7MD

[Zenogias]

I think that's called back face,Try Ctrk+N to swap face or W --> Flip Normal.

Each triangle has one side that's called front face and another side called back face.
(Back faces are the meshes that you looked from inside object Ex.Cube).

[Mr Chappy]

@Zenogias is correct, some of your faces are facing inwards(the darker ones) whilst the other ones are facing outward(how they should be!)

However your mesh is the real issue and cause of the problem. As such the std fix of recalculating your face normals(flipping their direction so they all face outward) won't work.
You have many N-Gons(faces with five sides or more) which should be quads(four sides), you also have overlapping faces on the rim of the cylinder(the little black dots indicate the center of a face, if you see one of these overlapping an edge you know you have a problem).

The easiest way to fix this is to delete your tube/cylinder to start again, then do the following :

1) Go to user prefs(File -> User Preferences). Select the "Add-ons" tab.
Scroll down until you find "Mesh : Inset Polygon", "Mesh : LoopTools", and "Mesh : Relax".
Enable all three of these add-ons. I am assuming these are all now bundled with Blender as default if not let me know and I will tell you how to find the missing ones.

2) Create a new cylinder via "Shift+a" -> mesh -> cylinder.

3) Select the two end faces, press "i" and adjust the inset until you are happy with it(this will be the thickness of your tube's rim) then press LMB(left mouse button).

4) Now press "delete", then choose "Only Faces". This will delete the two N-Gon faces in the middle of each end of the cylinder. Leaving you with the two holes, one at each end of the cylinder, which will form the open middle of your tube.

5) Now using either “vert” or “edge” select(not face!), select the two loops that form the newly created holes. Do this the quick way via :
"Alt+LMB" to select one ring. Then use "Shift+Alt+LMB" to select the other ring, “shift” keeps the first ring selected when selecting the second.

6) Extrude the edges by pressing "e" then press RMB, not LMB! Using RMB places the new extrusion over the original loops rather than somewhere else.
You can now scale the extruded edges to meet in the middle of the cylinder via :
Pressing "s", then press "z", then press "z"(makes the scaling local). Now scale(with the mouse) so they are close-ish to meeting at the middle. Press LBM to confirm, then in the "Resize" options window(bottom left hand side of screen) enter the value of 0 (zero) into the "Z" vector box and press "Enter". The two edges will now meet at the inner center of your tube.

7) Under the "Tools" menu find "Remove Doubles" click this and you should see that 32 verts have been removed(if you created the std cylinder) and that the two inner sides of the cylinder are now closed(joined).

8) Press "Delete" (with this join still selected), select "Edge Loops" and the central join will disappear.

This will leave you with a similar cylinder to the one you had made but made of quads, with no n-gons, no overlapping faces, and no Face Normal(wrong facing faces) issues.

[Jep]

Originally posted by Zenogias:

I think that's called back face,Try Ctrk+N to swap face or W --> Flip Normal.

Each triangle has one side that's called front face and another side called back face.
(Back faces are the meshes that you looked from inside object Ex.Cube).

flipping normals wont help :/

[Jep]

Originally posted by Mr Chappy:

@Zenogias is correct, some of your faces are facing inwards(the darker ones) whilst the other ones are facing outward(how they should be!)

However your mesh is the real issue and cause of the problem. As such the std fix of recalculating your face normals(flipping their direction so they all face outward) won't work.
You have many N-Gons(faces with five sides or more) which should be quads(four sides), you also have overlapping faces on the rim of the cylinder(the little black dots indicate the center of a face, if you see one of these overlapping an edge you know you have a problem).

The easiest way to fix this is to delete your tube/cylinder to start again, then do the following :

1) Go to user prefs(File -> User Preferences). Select the "Add-ons" tab.
Scroll down until you find "Mesh : Inset Polygon", "Mesh : LoopTools", and "Mesh : Relax".
Enable all three of these add-ons. I am assuming these are all now bundled with Blender as default if not let me know and I will tell you how to find the missing ones.

2) Create a new cylinder via "Shift+a" -> mesh -> cylinder.

3) Select the two end faces, press "i" and adjust the inset until you are happy with it(this will be the thickness of your tube's rim) then press LMB(left mouse button).

4) Now press "delete", then choose "Only Faces". This will delete the two N-Gon faces in the middle of each end of the cylinder. Leaving you with the two holes, one at each end of the cylinder, which will form the open middle of your tube.

5) Now using either “vert” or “edge” select(not face!), select the two loops that form the newly created holes. Do this the quick way via :
"Alt+LMB" to select one ring. Then use "Shift+Alt+LMB" to select the other ring, “shift” keeps the first ring selected when selecting the second.

6) Extrude the edges by pressing "e" then press RMB, not LMB! Using RMB places the new extrusion over the original loops rather than somewhere else.
You can now scale the extruded edges to meet in the middle of the cylinder via :
Pressing "s", then press "z", then press "z"(makes the scaling local). Now scale(with the mouse) so they are close-ish to meeting at the middle. Press LBM to confirm, then in the "Resize" options window(bottom left hand side of screen) enter the value of 0 (zero) into the "Z" vector box and press "Enter". The two edges will now meet at the inner center of your tube.

7) Under the "Tools" menu find "Remove Doubles" click this and you should see that 32 verts have been removed(if you created the std cylinder) and that the two inner sides of the cylinder are now closed(joined).

8) Press "Delete" (with this join still selected), select "Edge Loops" and the central join will disappear.

This will leave you with a similar cylinder to the one you had made but made of quads, with no n-gons, no overlapping faces, and no Face Normal(wrong facing faces) issues.

damn :D