There’s the code itself:
public final static Color red = new Color(255, 0, 0);
public final static Color RED = red;
The upper case letters were introduced in JDK 1.4 (to conform to its naming convention, stating that constants must be in upper-case).
In essence, there are no difference at all (except letter casing).
If I want to really be brave, Oracle might go wild and remove constants that is lower-cased, but then that would break all other code that’s written pre-JDK 1.4. You never know, I would suggest sticking to uppercase letters for constants. It first has to be deprecated though (as mentioned by Andrew Thompson).