Meazure - What are the dimensions of that image? How big is this window?
Just Color Picker - Free portable colour picker and colour editor for web designers, photographers, graphic designers and digital artists. Supports Windows and macOS.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Instant Eyedropper - Identifying the color code of an object on the screen is usually an involved, multistep process:...
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.
gcolor2 - GTK+2 Color Selector