The back of N64 cases are originally all the scales of orange/yellow. So you have to either
1) Greyscale it as per Grumbleduke's post, or
2) In Photoshop,
- Select the layer.
- Press CTRL + U
- Find the box marked "Colorize" and check it
- Match up the colors as closely as possible
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On another note
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I have 3 manuals (to my knowing ATM) for the N64. Super Mario 64, Paperboy, and BattleTanx. These are the colored versions of the manuals and where it states "instruction bookley/manual" is quiet far away from the center of the picture / can be easily avoided. Seeing how these manuals/booklets are already in a vertical format, would this possibly be an idea to make some vertical cases with alternate pictures? After scanning my N64 boxes, ill scan the manuals, however I don't know how I should upload them.
As some of you might tell, I am really in the N64 mood all of a sudden... tf... =D, ANYWAYS, here is my idea with the instruction manuals -->
Converting this manual into this case was basically the same as eating skittles; painless (for the most part). Actually, I'm angry when I eat skittles because they are gone too fast! Ramble oops.

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or

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The other manuals (thumbnails)



-Stoled Shenske's Mario covers for the illustration.
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Edit
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Those are all the games I have "colored" instructions for. After seeing the cases above, is there any interest in me uploading them as raws or whatnot?