Refactor.

[ViacheslavSoroka]

Code below:

if(rgba[3] == 0) {
    CGFloat alpha = ((CGFloat)rgba[3])/255.0;
    CGFloat multiplier = alpha/255.0;
    return [UIColor colorWithRed:((CGFloat)rgba[0])*multiplier 
                                          green:((CGFloat)rgba[1])*multiplier
                                            blue:((CGFloat)rgba[2])*multiplier
                                          alpha:alpha];
}

could be rewritten as:

if(rgba[3] == 0) {
    CGFloat alpha = 0/255.0;
    CGFloat multiplier = 0/255.0;
    return [UIColor colorWithRed:((CGFloat)rgba[0])*0
                                          green:((CGFloat)rgba[1])*0
                                            blue:((CGFloat)rgba[2])*0
                                          alpha:alpha];
}

or:

if(rgba[3] == 0) {
    return [UIColor clearColor];
}

Also, there is no need to do type casting to CGFloat. Type will be inferred automatically because of divisor 255.0.