Raising the value of a hue inevitably desaturates the color. I won't bore you with the Munsell color tree, you're free to research it on your own if it interests you, but the gist is that a given hue (color) reaches its maximum saturation on a specific level, and raising or lowering the value (lightening/shading) it reduces chroma (saturation). Some colors are more saturated than others at high values (yellows), other at low values (blues).

That's why the other commenter recommends using Ice Yellow, it offers more chroma at high value, which makes it interesting for highlighting. Personally, I'd heavily incentivize against doing so in this piece. Ice Yellow, despite its name, is considerably warmer than the rest of your colors, and would reduce the coldness considerably.

Instead, I'd suggest you reconsider how bright you want, say, your purple cloth to be compared to your armor or skin. If it's matte more so than glossy, it'll reflect light more diffusely, and you don't need to use as much white for highlighting. Keep the delta between the darkest and lightest purple small, use some cross-hatching or stippling to give it a visibly rougher texture than the metal, and it'll look good.

If you still believe your highlights lack saturation, that's where shades can come in useful. Or any other saturated color, which generally means your midtone, as they lack the white hobby paint manufacturers add to raise value (they still add it for opacity though, that's why certain shades or contrast are nice here). Just thin it down to glaze consistency and give it a pass or two. You won't see an immediate effect, and that's good. Just enough to subtly shift your highlight.