Have you looked at the DR400 textures supplied in the SDK? It was looking at those that helped me to work out what to do.
That does not look right for a texture with _illumination suffix.... more like a _light one. Both the illumination and the light texture are involved with how the object looks in the dark when the lights are on. It shouldn't have any effect how it looks in daylight.
To be specific about those two textures... the illumination texture is used to apply a diffuse lighting effect to the cockpit interior, and requires a point light to be set up for it in the TMD. The light textures is for lit up labels on gauges, buttons & dials etc, which are switched on by the panel light. I didn't bother with using the illumination, I just used the light textures, to light up all the dials and buttons etc for night flying... and that suited the Military style aircraft I was working on.
More critical to how the object looks in daylight are the color, ambient, normal, reflection, specular and specular alpha textures:
Color.... obvious this is your repaint texturing
ambient... Ambient shading application. I really like how this make things look. Its a lot of extra work because everything needs mapping out single space and then rendering the AO map for each object... which can take quite a while... I think my Spitfire fuselage object took about a half hour. When the grey scale AO mapping is done it can blended over a white background and you have your ambient texture.
Normal.... bump mapping of panel lines, Rivets fabric, wood and surface texture. Created from a greyscale (but RGB) texture by a photoshop plugin in my case (but other software options are available for that). I only use this for external objects. It does not work for me on internal surfaces or transparent items like canopy glass.
specular and specular alpha... greyscale textures work together to produce shine. On Spit both where very dark to give matt finish. It seems to me darker grey is less shine, lighter grey more shine. These snippets from another model so you can actually see something...
specular alpha
reflection... This texture creates reflection when sunlit. Greyscale blended over a red back ground. Lighter, More Red and saturated more reflection. Darker, less red less reflection seems to be rule. Here is a snippet from Navy Buffalo where I wanted paintwork matt and metal shiny reflective..
light... mostly a black background with details in red or whatever colour you want the labels lit up in
Anyway hope this is of some help
KR Matt