Multiple graphic per t-shirt (i.e. white on blue t, blue on white t)

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I am sure this has been covered since every store seems to have it.  I use custom cat and each product is imported individually.  I would like to have the same t, have a different color  image so I do not run into a dark image on a dark shirt and vice versa.  Example  The Gray and the Charcoal shirts have different colored images.  I would prefer not to have to manually create a variant for every single shirt. It seems as if this could take forever.   Is there a simple app for this or even better, can Shopify merge products.  Any help is greatly appreciated!