InDesign can do it.
Google Slides can do it.
PowerPoint can do it.

This would be so useful for groups and layers!

I agree that this would be a helpful feature and I am surprised that this isn't something Adobe has in Illustrator at this point. It has been requested on numerous forums and seems like it would be a basic feature at this point. I think it is lame that they tell us to draw lines for that purpose and just color them. There really isn't any good reason to have a useful and easy to set up guide tool but not make it possible to make guides in multiple different colors since that is a regularly useful thing in design and art.

Agreed! Could do with this!

This would be a really useful feature for commercial design work. I do a lot of design for panel art and silkscreening on metalwork, and the artwork has to be precisely aligned with edges, holes, degrees of rotation of potentiometers, etc. It can get really messy if there's a lot of features to deal with. If I'm zoomed in close, or there's two guides very close together, I can lose track of which one is which.

If ruler colours could be individually defined or coloured by layer that would be extremely useful and provide a much tidier and more readable workspace.

This may not be the solution you are looking for, but until Adobe addresses this, perhaps creating your own "guides" with the pen tool in their own layer would solve this? The line-guides can be any color you want.

SInce I am working on several different artboards at the same time, it would be great to be able to have the capability to have different color guidelines. I Also have times when using a single artboard it would be nice to be able to have more than a single color guideline so I can for instance a Color to guidelines for each artboard. There are times I need to use multiple guidelines and being able to have multi colors for the guidelines. I would be able to have more than one color at a time designated for different reasons