Standard import will strip the formatting - text only.
You can try to script it as Copy and Paste with AppleScript, if you are on a Mac.
I would investigate an option of saving Excel as XML and importing into FileMaker using XSLT. Although, you get only text, but you can import formatting as text as well, parse and apply it through the script.
To my knowledge there is no one-step procedure for this. Nevertheless, it's possible:
Use an intermediary format like html for import through a plugin, parse imported text for creation of records and sorting data into fields.
FM website lists some plugins in the "Solutions/Plug-ins & Integration" section.
YOu might check out the XL functions of the MBS plugin
I tried copy & paste on Windows, FM lost (at least) color (in some case, seems always on 2nd line of a cell data) when pasting.
The issue is already reported.
It seems FM use 2 steps when paste, "paste" "style", so applying only 1st style in copied data.
Hmm, it seems not a bug on FM at least when copy from Excel.
Excel's use of clipboard is not normal, if paste on Excel using enter key, copied things are vanished from clipboard.
Another case, copy on Excel, paste on Word, seems all styles are pasted, close Excel, paste on Word, some styles are not pasted.
Maybe, Excel coping to clipboard some "reference" to the documents, then after closing document, it will not referable.
Thanks for all the suggestions folks. Have not found a combination that works yet, but I need to explore the plugin market a bit. Will keep you posted as to what I find. There has to be a way to do this.