I also have a 30 year old LED clock, but it doesn't seem dimmer now. It has a high/low brightness switch that has always been on low, so maybe if you keep the current low they don't fade as much. (I think Ron tried to say this.) There is also no discernible difference between the segments as there would be if they had in fact faded a lot as some of the segments are on less often than others. ( I have seen this effect on other displays.)
I have also experimented with RGB LEDs and can say they are tricky to adjust. But the big problem is that the colors do not blend nicely; you just get splotches of red,green, and blue. Before ordering any parts you should do some research to see if anyone has solved this.