I assume you saved the files in jpg? If so, the only way to add color back is one of the AI "colorizer" routines and it won't be fully accurate. IF you saved the files in RAW, then all of the original file data is still there and Canon's DPP will display the proper color.
But if the camera was set to save only JPG files with the picture style set to monochome, then the color data is gone.
Check the menu system, go to tab 3 of the Camera (red) which is the first set of menu tabs after you bring up the menu. See what you have set for picture style, it is probably monochrome.
One of the first changes I make when I buy a new Canon body is to set the file save type to RAW. Yes, you do have to convert them in order to share jpg files but it is very easy with the free Canon DPP software and you don't have to worry about setting picture style, white balance, etc. when capturing the image and having the complete RAW data allows DPP to recover from significant issues (i.e. over or under exposure) at the point of capture. Your 90D is a nice and very capable camera body but it can provide so much more if you save images as RAW data files and then convert them to jpg.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
