How can I convert a greayscale to a colour of the same value?
It's not clear what you mean. There is no causal relation between a color and it's brightness/ luminance, so there can be no simple formulaic way to assign colors based on greyscale values. It all depends on perception or technical parameters like Gamma, an image's intrinsic luminance, color profiles and how the values were derived/ generated in the first place. You need to explain better what you actually want to do.
This is a project for a textile pattern, the original pattern is being designed in grayscale. Once the grayscale pattern is finalized colour veriations the pattern need to be created, so we need to keep the same hue but add colour.
thank you for your help,
You may try Edit > Edit Colors... > Recolor Artwork and change Hue and Saturation to keep the Brighness.
Good to hear that helped.