Does anyone know whether it is possible to create ROIs by combining all the pixels with defined RGB values (cHex)? Is there some workaround with the Color Threshold, by chance, or any better option?
that depends a little bit what exactly you want to achieve.
If the images you finally want to analyze have a variable color content the color threshold will most likely not give you reliable outcome since you need to specify fixed values.
If you want to use the color threshold, you can easily record the extraction of a previosly specified color/saturation/brightness combination using the macro recorder. On the resulting binary image you can run the function
run("Create Selection");. This will create an overall ROI from the extracted pixels.
I hope these short hints help you but if you need a more detailed description, post again potentially with some more details and an example image.