It's always a pain to scan watercolors. The translation from pigments to RGB is frustrating. Going from pigments to light is not an easy process. First thing to do should be getting out of that awful SRGB thing, but since sometimes it's impossible, let's do something about the picture.
First I would check the histogram. The histogram is a visual analysis of the percentage of black and white, or the percentage of colors , in a picture. It's very helpful because sometimes you feel like you are seeing something and actually there is something else in the picture. Let's see.
It tells me from Black to White that I have a lot of whites.
Doesn't ring a bell? A lot of whites means a light gray background. There's no real black, well fine I don't see any in the picture.
Then if I watch the color histogram , I'm surprised to see that my reds are really light, and that my paper indeed, is more gray -yellow than white. There's a lot of unexpected blues and greens...maybe what I see red is actually a brown.
Now I know what to fix to see what I want. And what I want is clean white and sharp red.