This may be a little late, but here's my answer :D
I'm not sure if there's an easier way, per say, but after looking into it, I think you can do the following.
In Paint.NET, you can expand your Color palette with the More>> button, this is the first step to finding the equivalent color-shifts for another color, let's say a blue. You will now see the RGB & HSV values. We won't concern ourselves with the RGB values.
Red
Taking your original color of #F80021, I created a lighter (#F77B8C) and darker versions (#CC3347).
What we want to focus on are the HSV values. Below are the HSV values for your original color:
- Hex: #F80021
- H (hue): 352
- S (saturation): 100
- V (value): 97
Now for the lighter shade.
(NOTE: this is just an example, replace this Hex# with your own, lighter-color version)
- Hex: #F77B8C
- H (hue): 352
- S (saturation): 50
- V (value): 100
And, finally, the darker shade.
- Hex: #7F1F2E
- H (hue): 352
- S (saturation): 75
- V (value): 50
Notice that the hue values stayed the same throughout the shade changes.
Blue
To create a blue theme with similar shades/values as the rest, you first pick a blue. In this case, #0000F8. Again, the HSV values are as follows:
- Hex: #0000F8
- H (hue): 240
- S (saturation): 100
- V (value): 97
Notice, that this blue's Saturation and Value amounts are the same as your original red's Saturation and Value amounts. The same will be true for your lighter and darker blues. (The lighter blue's S & V values will match the lighter red's S & V values, while the darker blue's will match the darker red's.)
Lighter blue.
- Hex: #7F7FFF
- H (hue): 240
- S (saturation): 50
- V (value): 100
And, the darker blue.
- Hex: #1F1F7F
- H (hue): 240
- S (saturation): 75
- V (value): 50
Conclusion
So, in short, for your new theme to match your old one, your new Saturation & Value amounts need to match your old S & V amounts and for your new theme's color to stay consistent, your Hue value needs to be the same in your new theme.
I hope this made sense. If you or someone else has a question, I'll do my best to answer it in the comments.