Hi,I am using Lightwave mainly as a modelling tool.I export the models as .obj so I they can be used in other apps.
These models are plain grey,is there a way to give a colour to the different parts?,a .mtl file has been mentioned to me but I dont know how to make one!.

  http://www.sharecg.com/pf/full_uploads.php?pf_user_name=jedswindells

Here are some free models ...

  Cheers! Jed.

(also posted at Lightwave forum)

I don't know Lightwave, but normally the OBJ exporter should write a .MTL file when you export the model, that contains material information the OBJ format is capable of expressing (things like color, specular, image maps, bump maps etc).  If it's not doing that, then you probably should check with some Lightwave help to see how to get it to do write MTL files whenever you export to OBJ format.

Given that most apps into which people are going to import OBJ files allow for much more sophisticated material settings than are conveyed by an MTL file, the important thing is to have enough appropriately define material zones on the model.

For those apps that can reproduce LW material settings properly when inporting LWOs (not many), then it depends how much effort you want to setting those up.

lightwave 9.6 does make an mtl file when exporting as obj, but any material node attribute will not export, and some "standard" material attribute will export, but will not look exactly the same. You should stick to pretty simple material settings in your modelling app, and tweak your materials in your "final" app.
One other thing. You need to have a bitmap texture in the diffuse channel for lightwave to export uvmaps when exporting to obj

If you want to make a material file for your OBJ, it's pretty simple. Both the OBJ and MTL files are plain text, and you can edit them with any good text editor. There's some good information on the OBJ and MTL file formats at www.wotsit.org, but here's the basics:

In your OBJ file, you need to add a line that looks like this:

mtllib FileName.mtl

Most export programs that create the file for you use the same name for the OBJ file and the MTL file. You'll put your materials in the MTL file. Still in your OBJ file, you need to add a line that looks like this:

usemtl MaterialName

above the faces that use that material. it's pretty common to have one material per group, and the "usemtl" line usually appears right after the group identifier, which is a line that starts with just the letter "g".

Your MTL file might look like this:

newmtl MaterialName
Ns 100.0
d 1.0
illum 2
Kd 1.0 1.0 1.0
Ka 1.0 1.0 1.0
Ks 1.0 1.0 1.0

The "newmtl" command identifies the material, and any program that can read the MTL file will mathc the name identified on that line with the name used in the OBJ file on the usemtl line.

In this example, "Ns" is the specular highlight percentage, "d" is a dissolve ratio designed to fade materials from one to the other across a polygon, "illum" refers to the illumination model, which I don't think any current program actually uses, "Kd" is the diffuse color (color is RGB, spearated by spaces, with values from zero to one), "Ka"  is the ambient color, and "Ks" is the specular color.

You can specify a texture map for the colors with the command map_Kd (and map_Ka, map_Ks, and so on) and a file name. There are lots of other commands to, but you can find them in the MTL spec at the web site above, if you're interested.

Note that there may be features in the MTL spec that your rendering program doesn't use, so you may have to experiment. As mentioned before, it's usually best to lay your materials down in the rendering program, and not try to export them from the modeler.