I think the threads you read were meant for replacing textures with re-coloured versions of the same texture. Sometimes 3D programs will automatically 'map' an object you create with UV co-ordinates to make life easier for you so it may seem like you can just replace colour with textures, but in general you need to map the textures onto the model before you export it to pepakura, (in just the way you did) pepakura has no ability to do this at all.

It is the model that contains the data needed to texture a model, so no, you can't just replace colours with textures, in pepakura or any 3D program for that matter. Solid colours and textures do not hold the UV data needed to texture a model. I googled a tutorial, and it worked fine for them, but I'm missing something. So, HOW do I scale up that texture to make it appear the way I want it to on my drawing? (green cap, with 4 large white spots?) When I zoom in there are MILLIONS of those little things on the cap. Now the bottom is still plain and the TOP is goofy. When I returned to Pepakura I was able to select 'Material0' now, and replaced it with the same texture. Then I went back to my source drawing, selected the cap, and colored it red. I unfolded the model, then clicked 'Settings > Texture Settings' I selected the 'FrontColorNoCulling' option, specified my texture, and ended up with the picture you see. The cap and base were both left uncolored, untextured.

I was originally trying to use a complex model I designed, and failed, so I tried the mushroom you'll see in the picture attached. So, according to several discussions I've seen, it should be easy to apply a new skin to a model in pepakura. Model: a Mushroom with 2 components (cap and base) exported from Sketchup as.

Pepakura sketchup