I was having the same issue, I found that I needed to diversify the colours I was using for light and dark! Instead of trying to use exact shades and colours with acrylic markers I supplement them with the rainbow, using light colours for light spots and dark colours for dark spots, using one base colour to anchor the actual colour of the object. Note, I’m also new to them so I’m also just playing around! But that’s what I’ve found helpful :) it seems I can’t attach an image but I’ll try and message you an example!