Change Colour without Changing Value

[mymaloneyman]

Hi there, I'm using GIMP for graphics for a video game project, and I'm trying to make several different colour palette variations on the following palette in such a way that the colour changes without the alpha values changing. Basically, after colourizing, converting the palette to greyscale would result in the exact same greys. How could I accomplish this?

Here's the palette in Hex:

#111111
#818281
#a9aaa9
#d4d5d4