Most organs consist of more than one cell type. Fatty tissue is white to yellow. Muscle cells are red meat, looks the same in all mammals. As the heart is a muscle too, it's mostly red, with a bit of fat around it, and some shimmering white to almost silver connective tissue. Fasciae in general are really pretty. The single fiber is white, but in smooth organized sheets they can shimmer like pearl in pale rainbow colors.

White blood cells are translucent, only the red blood cells are actually red, platelets are yellow. White blood cells climb around in almost all tissues, so even a red muscle has some of those, and yellow-ish nerve fibers and the red-grey-white of blood vessels, it's a mix.

When you put any cell under a microscope most of them are translucent and barely visible. There are a bunch of organelles inside, filled with whatever this cell's product/purpose is. That's where the color comes from. Like chloroplasts in plant cells, just those are green, the rest is translucent. Few cells are so colorful that it's noticeable when you look at just one. The sheer mass of them makes organs as colorful as they are.

To really tell things apart the cells are dyed. There are different dyes/stains that attach to different parts of a cell. They even come in fluorescent. Some are as simple as binding to anything acidic or basic, others cling to just one specific molecule. Without dye it's hard to impossible to tell what is what.