I've made a little utility for batch converting hexadecimal RGB color codes to Construct "color values" (those negative integers you can pass to "set color" actions). It has a big textarea so you can paste multiple color codes at once (eg. you can select and copy them straight from Lospec), then it utilizes R0j0hound's excellent hex2color function to convert the values.
You can click "Make swatches", the generate color swatches that you can click to copy the color values to the clipboard. If that doesn't work on your browser for some reason, you can also hover swatches and use CTRL/CMD+C to copy the values from automatically focused & selected text inputs.
You can also click "Copy global constants" to generates global constant declarations for all colors and copy it to the clipboard, so you can paste them directly in your C3 project's event sheet.
fodi.github.io/construct-3-projects/ConstructColorConverter