Hey 'S', I just read through your post and have noticed you could be doing this technique a little better.
the background was drawn with pencil, scanned and coloured in photoshop. It is a layer set to about 15% transparency above the colours.
You shouldn't 'just only' set the opacity of the sketch layer.
I took your sketch and put some color under it.
A: This is your sketch ONLY set to 15% opacityB: This is your sketch with Multiply and set to 15%As you can tell the colors on A are lighter than B. B remain the colors that they are supposed to be, as A fades. (normally I wouldn't set the
Multiplied layer to 15%, because you can't see the sketch anymore. I only did that so you could see the difference in the colors)
I could be completely wrong and you may be doing this right. If I am wrong, shoot me.

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You know most of these steps, you're just missing the
Multiply step.
But for those of you who don't know this technique of how to sketch and color on photoshop it's quite simple.
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You scan the sketched image.
You place the
sketched image on a layer.
- With the sketched layer selected, go to the Layers window.
- There is a drop-down list. In that list you'll see
Multiply, select that.
Create a blank layer Under the sketch layer.
On the blank layer you draw the colors (with the Pencil tool; not the Brush).
You only have to follow the edge of shapes with the Pencil tool, then Fill the inside area with that color.
There you go! You have a basic colored sketch.
Multiply lets the dark colors of the sketched layer stay, while the colors on lower layers bleed through the white (of the paper).
Also if you don't like the sketch being as prominent, you can lower the opacity anytime even though it's set as Multiply.
Then if you want to add detail to the colors you use the Magic Wand tool on the colored area you want to add detail (this way you don't have to worry about crossing over the lines. Lets say you're adding a nice shine/shadow to that Stove, you don't have to worry about the Brush tool or whatever drawing over the back wall).
Create a new layer before adding detail (or what I do, is add a new layer for each object/area shading/lighting).
^ This layer must either be on top the colored layer, or on top the sketched layer. Depending on what you're doing.
You pretty much just play with the opacity levels of the Brush tool to get nice effects.
Some helpful links (in case I am as confusing as hell):
how to color a pencil sketchhow to color a photoshop sketch