If you've ever scanned in a watercolor drawing (or any image with soft hues and delicate color transitions) you probably know the horror of seeing your digital image looking VERY different from the original.
Where black-and-white images or strong colors are quite easy to edit, techniques like watercolor and colored pencil require more care. Especially if you prefer leaving parts of your paper white, but don't want the paper structure to show up in eventual prints or digital presentations.
So while I'm still new to color images and maybe not the best choice for advice, I'll show how I usually edit my watercolor drawings in Photoshop.
Photoshop can be purchased on a monthly basis, which is a bit of a curse and a blessing in one. While I hate paying every month instead of just 'getting it over with' once, I'm glad that this takes care of eventual program updates, and it makes the initial expenses a lot more manageable. While I've used a lot of different graphic programs, from freeware like GIMP to Paint Pro SAI, Corel Painter and Clip, I do come back to Photoshop for image editing all the time. It isn't the most intuitive program for actual 'painting' (that's where Corel Painter wins...) but since I prefer painting traditionally anyway, that doesn't bother me.
But these tips should be applicable to a lot of programs since I'm not using anything fancy. The basics are, well, basic enough to work nearly anywhere.
My other materials can be found on my 'Resources' page - from watercolors to scanner and graphic tablet, I've listed them all there. But as with Photoshop: The materials really don't matter all that much, as long as you know what you're doing. ;)
First, you'll need something to scan, of course.
Select the highest resolution that works for your laptop/PC. I work with 600 dpi since that allows me to zoom in while correcting the image - anything beyond 600 dpi is just a pain in the ... backside of my laptop.
Once you've got your scanned image open in Photoshop (or graphic program of your choice), crop it and duplicate that layer! Having a layer, set to invisible and pushed to the very bottom, is the perfect backup and comparison material for when you mess up along the way.
For a long time, I then went directly into color adjustments to get the image as bright and natural looking as possible. But I've changed my approach a few months back:
I apply a selection mask to my objects first.
It's a lot of work, yes, but I've found that trying to get the background white by color selection/adjustment layers only, I always use more color nuances than necessary in my drawings. Especially for my sticker motives I like to keep the 'hand-painted' feel and the editing to a minimum, so this gives the closest results.
Step by step:
Background Color Selection
Go to 'Select' and choose 'color selection', then click on the white color of the paper to give the program a color reference. This will select all white or white-ish spaces on the drawing, with more or less leeway depending on the percentage you enter. Play around a bit - this really depends on your paper, the intensity of your watercolors etc.
But really, don't fret too much if the selection isn't all that accurate. Because next, we'll expand that selection (by clicking on 'Select', then 'Options') by 1 pixel, thus swallowing some of the pesky dots that refused to be selected.
Then, inverse the selection (again under 'Select') so that instead of your white space, you have only the colored objects selected.
Create a vector mask by clicking on the tiny symbol at the bottom of your 'layers' window. This makes everything that isn't part of your selection transparent.
Working with Layer Masks
Now for the incredibly dull part: Take out your graphic tablet and refine that mask. This means both correcting the borders of objects and the interior of objects where you'd left white space or only very light colors in your drawing.
Really, my only advice for this is: Reduce the size of your Photoshop window and re-watch your favorite episodes on Netflix on the same screen. For reference: Correcting the mask for my Bonsai trees took two episodes of Parcs & Rec. Yay. Good times.
Yes, it's possible to get good, clean results without masking. But especially if you want to 'cut out' individual parts of the drawing (like individual trees) to make stickers, re-arrange the layout, or plan on printing anything: You'll be glad you did this later on. Our eyes see details like gray smudges or dots much better on paper than on screens, so while your picture might look clean on the monitor, you might be in for a surprise after printing.
Bonus Tip: Personally, I find working on a transparent background or white background incredibly hard on my eyes. That's why I created an extra layer in a dark color. Much easier to spot stray smudges this way!
Adjust Layer Mask with Brush Tool in Black and White
I tend to leave a slight border around my drawings, especially if I have used a pen instead of just watercolors. It's easier to keep the drawing's borders looking natural and hand-drawn instead of overly digital. But every image requires a different approach.
You can also see that now that my masks are in place and the clean-up well-advanced, I've started playing around with adjustment layers to see how my final pieces will look. I mostly adjusted brightness levels and cranked up vibrance, saturation and shifted the hue slightly down.
Color Balance comes in handy especially when working with colored pens to draw the outlines, like the sepia I'd used for these. Sometimes, they can look too bright or not match the colors in the aftermath. I toned down the yellow in those lines via Color Balance.
Apply Layer Mask
Once every tree is corrected, a right-click on the vector mask (the small black and white thumbnail in the layers window) and selection of 'apply layer mask' will make the mask permanent, leaving you with individual images on a transparent background: No paper textures except in the drawings themselves, no smudges, no dust grains from the scanner.
Then, I can select individual trees with the lasso selection tool, copy-paste them into new files and save them individually for future use - like arranging them on A4 paper for printable files, or smaller formats for sticker sheets. Here's where the initial 600 dpi decision comes in handy: I only need 300 dpi for my prints, which makes the trees large enough to work on A4 paper despite the original drawings being slightly smaller.
Is this the most time-effective way of getting a white background? Nope. But it is the way that keeps my original drawings looking as close to reality as possible, with me being able to adjust the background and the drawings separately.
I'm not sure if this 'tutorial' is helpful - I do hope so, but I think you'll need to know some Photoshop/graphic program basic knowledge, like how to work with masks and adjustment layers, to follow along.