The Power Palette: http://www.reapermini.com/PowerPalette/
The Power Palette is a fantastic aid for miniature painting! Upload an image, then click inside it; the Power Palette will give you the Master Series Paint color or colors it thinks are either close matches to the point you chose or colors you can mix together to get a great custom color!
Paint Range Compatibility Chart: https://www.dakkadakka.com/wiki/en/Paint_Range_Compatibility_Chart
Unable to find a paint that you used to use? Has something been discontinued? Want to find the equivalent paint in a different range? Here is the ultimate painting matrix, comparing the games workshop Citadel paints with every known equivalent. Please note, hex codes are all approximate – double check before you get a batch of paint made up to match that colour!
PaintRack (currently only for Android): https://play.google.com/store/apps/details?id=com.courageousoctopus.paintrack
Allows you to track your paints with over 5000 paints from top manufacturers! This is a great to keep track of what paint you used on which miniature. You can follow the 2 developers on their Twitter account here: https://twitter.com/courageousocto .