the way these kinds of packs work is that every block is randomized and the texture selected depends on the coordinates of the block, so without modification there's no way to control the textures.
if you know how to edit, you could dink around in the files and remove the texture files you dont want and remove references to those textures in the randomizer code

I just made one for melons, inspired by this texture pack. www.planetminecraft.com/texture-pack/colorful-melons/

I have not seen a pack like this, but with melons. I think I can probably make that, so thank you for this idea!

I have never heard of blue pumpkins...I googled it now and it looks like a gray pumpkin, that has already been added. But maybe I will add a bluer version in the future.