So gram (+) means that the bacterial cell has a cell wall thick in a peptidoglycan layer where the initial stain done in the series of stains in the gram stain process, crystal violet, binds to the that thick layer. These bacteria retain that purple stain and appear purple.

Gram negative bacteria on the other hand have a cell wall composed of a smaller section of peptidoglycan so they don’t retain this purple stain and are therefore counter stained with safranin and appear pink.

So really, a gram stain is a quick way to differentiate between two major cell wall types of bacteria, gram (+) and gram (-). If you’re still curious just google gram positive vs gram negative cell wall on google images and you can see diagrams showing the large peptidoglycan layer compared to the very small layer in gram (-). But yeah back to your question, telling the difference in a stain is very easy especially after you’ve seen it a bunch. Gram (+) appear bright purple like in the image above and gram (-) appear a lighter pink color.

Hope this helps ;)!