Would you be able to see an colon/intestinal tumor on xray without any contrast?

Unlikely : Most GI tumor in the colon or intestine are not well seen without contrast on x-ray. Endoscopy is often used since an abnormality can be seen and biopsied.

Probably not.: The best way to see a colon tumor would be an endoscopy(look inside the colon). Because colon tumors are the same density as normal tissue, unless the tumor was large, contrast would be needed to define the mass.