When the Alpha value is above zero and so not fully transparent, the RGB colors do matter. Here is FFFFFF44 on the left and 00000044 on the right. The latter has more solid borders than FFFFFF00, and it has smoother borders than 666666.
Contrary to what I wrote below, GD will now makes the background transparent for any background color that was not fully transparent from the beginning. This keeps it from recoloring the whole space when using an Alpha value above zero.
When the Alpha value is above zero and so not fully transparent, the RGB colors do matter. Here is FFFFFF44 on the left and 00000044 on the right. The latter has more solid borders than FFFFFF00, and it has smoother borders than 666666.
Contrary to what I wrote below, GD will now makes the background transparent for any background color that was not fully transparent from the beginning. This keeps it from recoloring the whole space when using an Alpha value above zero.