Can I use it on old saves?

Thanks to LCStark, since the version 0.1.2 you can now use the mod on old saves and new saves alike!

Can I edit the colors?

Yes! Simply download the zip file, go inside and find the colorblind.lua file. You can edit it with notepad++.
You will find two lines of code

colorblindness.chart.pollution_color = {r = 29, g = 0, b = 255, a = 0.8}
colorblindness.chart.default_enemy_color = {r = 255, g = 255, b = 255}

One uses RGBa and the other simple RGB. You can use these websites respectively to change the colors, just update the code and you're done!