There IS NO COLORING-ONLY software out there that is good enough for any production use. XD However, I'll try to help where I can.
For me, Krita does WONDERS (though I use it on Ubuntu, mostly)! There are many different ways of coloring with Krita, and it takes a while to figure out the techniques for each purpose, but I usually stick to the tried and true, painstaking method: using the brushes and hand-painting things in.
However, since you mentioned Photoshop, I'm assuming you're using Windows. So if you already have Photoshop, then brilliant! You may not need anything else at all! Since you're using mouse only, then just use the Pen-tool to create your colored forms.
OR you could get Paint Tool Sai, since that has been many artists' tool of choice when it comes to coloring. I remember using it in times when I didn't have a tablet, and it worked smoothly for me (though I had to be a bit cunning to make it work for me; see, it doesn't have a pen-tool, so I had to use "vectors" in the way traditional animators use -- Black outlines for outlines, Red for color-shade borders, Blue for Highlights, etc. -- and put in the colors in those regions with Bucket tool, and remove the Blue and Red outlines later).
But if you really just want cell-shaded look, like anime does, then you're better off simply using Inkscape's vector tools. If you want to do line-arts as well, you'll learn coloring with it. However, if you already have a pre-sketched art that you want to color, here's what you do -- a white block on the bottom layer, art on the top layer, and coloring taking the middle layer. Of course, the art layer wouldn't let you see the coloring layer, so you simply turn that art layer to "Multiply" mode, and start coloring on the coloring layer!
Hope that helps. Happy coloring!
