As far as I know, there is no standard color scheme other than that which you pointed out in Excel. Excel more or less set the standard here, and their biggest competitor Google Sheets has not put forward a different set, so I'd say these colors continue to hold.
An item being "greyed out" has become a common visual cue in UIs for something that cannot be changed, for instance form fields that are dependent on some other condition being enabled. Grey text also blends with the default white background, making it stand out less, which can be problematic if you want the output data to be the focus on a report, which you probably do. Therefore, Excel design seems to try to split the difference by combining a grey background with bold text for the Output and a popping orange for a Calculation (and the difference between Output and Calculation is not clear), thus letting you know that while this data should not be changed, it is still important!
The Excel colors are probably a good place to start, but if you're making a spreadsheet that relies on a lot of human input where you need to point out inputs and calculations, you should try to use other visual cues as well, including placement on the page (inputs at the top and left lead to calculations on the bottom and right), cell borders, even things like forcing inputs from a defined list so that users will see a caret to select a list. If the user does not need to see the intermediate calculation steps, you can hide these cells or put them on a hidden sheet. Finally, do not rely on the greyed out cells to point out that you have formulas that shouldn't be erased: lock cells and sheets for editing to avoid accidental deletion!