It is always preferable to grade from the original raw files but , if this isn’t possible then a clean Prores 4444 or DNxHR 444 export would also work.
After a rough cut/editing, flatten and simplify your timelines (Duplicate your timeline as a backup before simplifying your timeline).
A flat timeline means that your timeline only contains one or two video tracks with basic cuts (in/out) and nothing else.
In the simplified timeline, remove all the transitions (can be added after color grading), reset any custom crop/framing, remove stabilizations, remove nested sequences, text/graphics or things does not need color grading/correction process can be removed. Any empty tracks should also be removed. Any retiming/speed ramps can be removed before grading and can be added back later.
Export the XML from your NLE. XML is a lot more detailed of a descriptor than EDL. It will often take things like sizings, transitions, time remapping and so on with it as well that EDL won’t. These days EDL files have been replaced with XML-based files which include more information.
It’s worth noting that Premiere Pro can export to a Final Cut Pro XML, which contains more information (retiming, cropping, etc.) than a simple EDL file. And coloring software like DaVinci Resolve is able to import those XML files pretty consistently.
For color grading, provide a brief summary of camera model(s), color space and gamma (for eg., ARRI/Alexa/Log C or BMPCC 4K/Gen5/Film etc.,) used for shooting to allow for a more accurate grade project setup. (this is especially important for transcoded footage Prores/DNxHR etc., since the metadata would not be available after processing)
If you have any reference images/shots you would like to match please let them know.
During the process of color grading reviews of a color pass can be done over services like Google Drive/Dropbox Replay/Frame.io
When you speak to your colorist, make sure you let them know your export requirements for colored footage. Ideally, you want something as close to the original format as possible, so that you have greater flexibility when choosing how to export the final delivery file.