Change alpha component in a function returning a float *color

Question

I've got a function returning a (float *) color. It works, until I tried to change the alpha component of one color, and now almost all colors have alpha issue, even if the color don't pass the test "if (alpha)".

eg:
float * colorTest = getColor(0); // don't give me the good result anymore, once any getColor(1) is called before.

float * getColor(float alpha) {
    float * col;

    if (val == 1)
        col = colorWhite; // colorWhite is a float *
    else if (val == 2)
        col = colorRed;
    else if (etc..)
        col = colEtc;

    if (alpha) {
        col[3] = alpha; // this doesn't works
    }

    return col;
}

I also have tried to change the alpha value through the col pointer, with same result:

if (alpha) {
    float *a;
    a = (float *)(&col + 3);
    *a = alpha;
}

Any advice?

Edit

Got it to works, thx to hyde who told me that I was changing my global colors. (I was used to falsely considered float * as array) So, I just copy the result in a second variable and then modify this new color.

float * getColor(float alpha) {
    float * col;
    float * tmp;

    if (val == 1)
        tmp = colorWhite; // colorWhite is a float *
    else if (val == 2)
        tmp = colorRed;
    else if (etc..)
        tmp = colEtc;

    memcpy(col, tmp, sizeof(float)*4);

    if (alpha) {
        col[3] = alpha; // this now works without issue
    }

    return col;
}