I would like to suggest a color replacer brush. Yes, the “Replace Color” function exists, but that replaces all of a color in a given image. It works quite intuitively as a brush. This feature existed in an older program I used to use for my pixel art (Paint Shop Pro 9) and was one of my most commonly used tools.
Here’s a demonstration. Pay attention to the color palette on the right. Watch as I draw on all areas containing the background color I picked using the foreground color, without touching any other color.
Drawing while holding down right click instead of left click simply reverses the foreground and background color.
You can also double click to perform a quick equivalent of Aseprite’s “Replace Color” function:
What is the use case, you might ask? Maybe I want to replace some small highlights on a character’s gloves. I can’t just do “Replace Color” on the highlights because the same color is being used elsewhere. The fill tool is impractical because it’s multiple small pixels, I would have to click several times. With the color replace tool, I can quickly go in and replace the highlight without affecting the rest of the image.
Or say I want to do some quick highlights on his hair using a big brush, without affecting the lineart, face, or background:
Of course, these functions can be approximated with clever use of the select tool, but it’s often time consuming and requires switching between multiple tools, when a single color replace tool is much more intuitive and time saving. Ideally it would have the same functionality as the existing pencil tool, with changing size and shape. Or it could be a tickbox option for the existing pencil tool, “only draw on background color”.
If there’s something like this already existing in Aseprite, I’d love to know. I’m not even sure it exists in later versions of Paint Shop Pro, as I stopped buying the most recent versions. It’s one of the only things I miss, and it’s ideal for building upon an existing palette or making quick tweaks, especially on pieces that have a lot of dithering where it’s time consuming to use the fill tool/magic wand.