Welcome aboard.
Adding some raw plastic and re-heating is the best route to follow. Depending on the color that you want to end up with by adding more raw plastic, you may want to add a shot of stablizer just in case so you aren't burning the color. We've all had this happen and we've all tried the fixes, but only a couple really get the job done right. Now if this were me and knowing now that its easy to overwhelm a color with a darker color, I'd let this current batch cool, bag it and save it for later. Then I'd start over. You can always come back to this later on when you've developed some color adjustment skills. Until you know it is only good as black, assume it can be ressurected and this "iffy" stuff can be a great learning tool down the road.
It is best to mix your primary color into the raw plastic before cooking. Once you get that raw plastic heated to about 320 degrees you can add some of the black, very small amount mind you, to bring the color to where you want it and finish the cook to get to the needed 350 degrees. Flip a mold over so you have a smooth surface and put a nice drop of that plastic on the mold and let it cool just a second until you can handle it and check your color under a good light or take it outside where the sun can light it up. If its good enough for you, pour away. If it still needs to be toned down with some black, use that toothpick trick and stirr it up good between the additions so it doesn't get away from you.
Color isn't an automatic thing and there is a learning curve you'll go thru. Some over-doses can be expected. How colors play on another comes with experience and the more you work with the colors the faster you'll master this stuff. You're already on your way.
