Hi PGD people

I bumped into a Transperancy problem.
I am currently using a 24 bit texture in my game with $000000ff (bleu) as transparent color.
The backbuffer format is D3DFMT_A8R8G8B8 and
i always use the backbuffer format for Texture initalisation (Just thought its the best way ).

This works fine, but i want my engine to also use 16bit graphics. I think that is D3DFMT_R5G6B5.
But when i Initalize a 16 bit Texture,i have to pass a different transparent color because it has to be 16bit
(I though of $001F but it doesn't work ).
Maybe the actual data of the texture is changed, since its a 24 bit Bitmap loaded on a D3DFMT_R5G6B5 texture, so some color data might be lost. :?
I have not been able to find the correct value, so far. :cry:

Does anyone have a solution or a good color conversion routine?