Vertex colours no longer exist the way they used to. They were replaced by Color Attributes. You can access them in the Object Data Properties tab(The same one that's open in the image) You just have to click on the + icon and a new color attribute will be created.
There will be a few options after you click the + icon.
Name: The name of the color attribute
Domain: This will decide wheher the Color Attributes are stored per each vertex or per corner of a face.
Datatype: You canchoose between RGBA color with floating-point precision or with 8-bit precision.
Color: This will the default color that is applied to each vertex/face-corner.
To access these in the shader editor add the attribute node and type in the name of the color attribute.
To make it visible in the viewport choose the attribute option in the color section. Make sure that the correct color attribute is selected(in case you have multiple color attributes)