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is there any app in which I can color the objects and exporting only the texture in any image format? The game engine on which im working on doesnt support colors within the files

Ummm.  Gotcha.  I've heard that before.  Does it trigger an error or just strip out the color and make everything white?

no error, it just imports the object complety in grey (the default color of imported meshes) with the  texture parameter empty

Then you can use Assetforge to add textures, export the results, then use them in your game

does assetforge allow to export the texture in png? 

I think so.  If not, export as obj, convert the texture to png and edit the mtl file to change the image file extension it's looking for.