Be cautious of still images some clients provide to you; they may be the ones they use for printing so use CMYK rather than RGB which is what FCP likes.
If the colors are WAY off or the image is just dark and no matter of color correction, brightness or contrast monkeying will bring it up to a useable level, check the color space of the image.
FCP won't tell you.
You can quickly find out two ways:
1. Get Info on the image in the Finder. Look for "Color Space".
2. Open the image in Preview and hit Command + I for the Inspector. The first tab will show the image's "Color Model".
To fix Save As… and RGB image in your favorite image processor.
Btw, a fast way to fix a whole mess of images (other than a batch process in Photoshop) is to use ColorSync Utility. What? (yes, I know it's from 2009…)
1. In the Finder hit Shift + Command + U to open your Utilities folder
2. Hit the C key to highlight ColorSync Utility
3. Drag your image onto it's icon to open it in ColorSync Utility (or right-click on it and use Open With…)
4. At the bottom of the window you'll see a pull-down, set it to "Apply Profile"
5. Then to the right of that in the middle pull-down select "Display" > "Adobe RGB (1998)" (or whatever you need)
6. Hit "Apply"
7. Save or Save As…
You can also make an Automator script/application.
1. Launch Automator which is in your Applications folder.
2. Click on "Photos" under the Library
3. Drag "Apply ColorSync Profile to Images" to the workflow window on the right.
4. Automator will ask you if you want to make copies of images you alter in case it screws up the originals. I choose yes, but that's me.
IMPORTANT: DO NOT CHECK "Preserve original color space" (that defeats the whole purpose of this Automator App you're making)
5. Save As… an Application. Name it.
6. You'll have your very own (super simple) RGB converter application on your desktop. Drag images onto it to convert them to RGB.
As it, it'll copy images to your desktop and that can get mess if you have a lot of images. Where you see "Desktop" in the top action you can select it to any folder you want to to keep things tidy.
What I do is to make a folder called something like "Converted to RGB", then tell the App to use that folder for the converted copies of the images, then place the App itself inside this folder. Then all I have to do is drag a pile of CMYK images to the app in this folder and watch it populate with converted still images. It keeps things neat and organized.
A way to be further organized -if you need to convert images often- is to place the "Converted to RGB" folder in your Documents folder and then drag it into the Finder's window sidebar. Then, drag the app you've made into the top of a Finder window for easy access. That's just one idea, there are myriad ways to organize this.
Another option is to make it a Folder Action or a Service rather than an Application, but I'll leave that for you to explore :)