You can select a pixel's color on your screen using the native eyedropper tool (not just the browser window) and modify the selected color in a popup view. This extension stores the last six selected colors and represents the color in "HEX", "RGBA", "HSVA", "HSLA", and "CMYK" formats. You can press a button to copy the selected color format in the clipboard.
Uses the native eyedropper tool that works outside the browser
Automatically copies the selected color to the clipboard
Has an advanced color editing interface
Image Files: If you want to grab a color from an image, open the image either on a browser tab or use a native image viewer (e.g. Adobe Photoshop), then press the "Eyedropper" icon on the popup to activate the global color picker.
Video Files: If you want to grab the color of an object inside a video stream, try to play the video and seek forward to the frame, then pause and use the eyedropper tool to capture the pixel's color. If your browser supports the format, you can drop it into a browser tab.