I was wondering whether there's a way to classify simple non-spectral TIFF images in ArcGIS (maybe based on similar colour values of image pixels like graphic software applications such as Photoshop can do)?
If the image is in RGB color, you can use image classification tool to extract some data using maximum-likelihood classification for example. You need to take training site for the area of interest. I use it sometime to extract some information from maps instead of digitizing. But you have to make sure that area you want to extract is clear enough.
You need Spatial Analyst to use image classification tool.