Basically, if you don't have a mathematical method (a forumula) for determining the what the R, G, and B values are for each case then it's not possible to not write them all out.
UNLESS, you are actually meaning that you want R to be random, G to be random, and B to be random (this will give you a random color every time). In this case, you can do this:
PHP Code:
rgbaura[0] = random(256);
rgbaura[1] = random(256);
rgbaura[2] = random(256);
Also, you should never assign a whole array using an equal sign unless it is when you declare the variable (when you use "new"). In your case, you should assign each of the three values as shown above.
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