You could look into control clicking a button on Excel. May or may not work nicely and easily.
For an actually good solution, I'd recommend looking into VBA.
Or if you want to use AHK look into using the Excel ComObject.
Here's a quick little AHK code snippet to turn the currently active cell red:
XL := ComObjActive("Excel.Application") ;reference our open Excel document
XL.ActiveCell.Interior.ColorIndex := 3
For VBA you'll find loads of tutorials etc with a quick Google search.
For AHK you'll find some as well, but maybe not as many.
Here's one for AHK that seems to cover a good amount of stuff, however, seems like you'll want to use his library for all that he does. I guess be easy if you just want to get the job done, but not very good if you actually want to learn how to do it:
https://www.autohotkey.com/boards/viewtopic.php?t=63032
EDIT:
Just realized Excel was the example, instead of the fill color button. So I'll add some more about control clicking and control sending.
Use some way to try to get the class, or even more preferably the handle, of the control you wish to click or input to. The controlclick/send AHK documentation pages give some example ways for this.
A possibly easy way could be to use WindowSpy.ahk (an ahk script that with the ahk installation, you'll find it in your install folder). Might also be named something like AU3_Spy.exe.
If you can't get a class or hwnd, you can use coordinates.
The documentation also shows this and has examples.