DON'T DO IT!!
Even if there is no copyright symbol, the works are still copyrighted. A creative work becomes copyrighted as soon as it becomes fixed in a tangible medium (and yes, that "tangible medium" includes computer files).
Before you use anyone else's work, make sure that you have obtained a license (permission) to do so. Otherwise you will likely infringe on someone's copyright and have bigger problems. If you are not familiar with copyright law and these kind of licenses, make sure that you have any agreement reviewed by an Intellectual Property Law attorney to make sure the license covers the things you want to do with the licensed materials.
This reply is not and should never be considered
"legal advice." If you need legal advice, consult
a licensed attorney in your state to make sure you
understand both your state and federal laws concerning
your issue. Your attorney will need specific and
complete facts to provide you with legal advice.
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As my colleague has noted, you're contemplating copyright infringement. Only the copyright rightsholder and their licensees has the right to make "derivative works" out of their original works.
Please see my answer to your other question - you need to consult an IP/business lawyer to make sure your t-shirt business follows the rules and doesn't expose you to lawsuits.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
To add to the other posts, which offer good advice, I will suggest something else on a different tack. The problem that you will face based on the example of use you gave is that you will be infringing the rights that other parties may have in those designs (e.g., trademark and/or copyright.) Simply changing the colors in a design is not "transformative" enough to get you out of a claim for copyright infringement. Really, if you were to incorporate other people's designs you would have to generally really transform the underlying design in such a profound fashion that the new design is no longer "substantially similar" to the underlying one. And generally speaking that is a very hard thing to do. Moreover, you would be surprised how many people get caught lifting designs from others. Just see the link below. That being said, it is almost always the best approach to begin from scratch and only take inspiration and ideas from others, not actual work product.
I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.