Most tumours are reddish-pink blobs, without much structure to them.
On this webpage there are some pictures of a tumour that’s been cut and stained with a purple dye. The image is from a microscope, so you can see individual cells: http://emedicine.medscape.com/article/1661577-overview
So, if the tumour was well progressed, you may see a central region of necrotic (dead) cells surrounded by the tumour mass – and then on the outside you would see many blood vessels which the tumour had actually grown itself. This is called ‘angiogenesis’. If the tumour were malignant, you would also see it invading into surrounding tissue, and it may have a star-shaped appearance. Benign tumours can be more rounded in shape.