A more complicated answer:
You can change the color of the raw material by editing the RGBA values in the .i3d file using a text editor.
The RGB values work like this -> 0=Black & 0.5=Grey & 1=White
The A values work like this -> 1=100% Opaque & 0.5=50% Opaque/Transparent & 0=100% Transparent
Here is an example <Materials> section from a simple hitch assembly for importing.
<Materials>
<Material name="Material #61" materialId="7" diffuseColor="0.811765 0.823529 0.505882 1" ambientColor="0.815 0.827 0.507" cosPower="10" specularColor="0.178 0.178 0.178">
</Material>
<Material name="_vehicle_truc01" materialId="8" diffuseColor="0.0784314 0.0784314 0.0784314 1" ambientColor="0.0784314 0.0784314 0.0784314" cosPower="10" specularColor="0.126 0.126 0.126">
</Material>
<Material name="_vehicle_truc02" materialId="9" diffuseColor="1 0.0431373 0 1" ambientColor="1 0.0470588 0" cosPower="10" specularColor="0.198 0.198 0.198">
</Material>
</Materials>
So, I could add 3 texture files to the above section and be done...However, consider this...Do I really want to make the game have to load 3 more graphics...Instead what I do and it may not be right or the best way but it works for me.
I have blender installed (but only use it for this task). Anyway once it opens I switch to "Texture Paint" mode which brings up a "color pinwheel" in the left pane of the screen. Then I can select the color I want and right below the pinwheel it will show you the color you have selected now click on that and it will show you the correct RGB values to enter in the "diffuseColor" and/or "ambientColor" fields above. Then adust the A value for Opacity. Note: "ambientColor" only uses RGB not A...
Short explanation on diffuse/ambient color....
diffuseColor lets the amount and angle of the light source affect the color ambientColor is not affected by the light.
There is a ton of information online about the differences.
The third set of numbers "specularColor" is short affects the shinnyness of the object....it's not how to make something chrome but you can add a shine to an object....I'm sure someone on here will clairify that....or any of this actually....

By changing the colors this way now I can import the above hitch assembley to any mod and I don't need to worry about making sure I got all the textures copied or to the right place....
Good Luck w/your project!