Depending on how heavily they were thinned could cause changes to the shade of paint John. The type of thinners also needs to be of a type that wouldn't attack the material of the structure of the model. Looking at a model in differing degrees of light, early morning, evening, indoors, outdoors and things do begin to take on a different hue. I agree with the gloss finish on models, however if you have painted the model yourself a gloss finish can be a great aid to adding different mediums of weathering. Of course when it is weathered you can give it a satin or dull coat to get the finish you require.
I read a great piece on using authentic paints from Ian Rathbone years ago and it was truly eye opening.
Rich,