change of lettering colour

As is often the case, it depends.  If you are seeing it as a linear gradient, then the easiest way is to use Foreground to Background (which is an existing preset) and apply it to the Type layer as a Layer Style.  However, this gives you limited control of the gradient, and I am not entirely sure whether it is linear or radial.  You can adjust the spread with the Scale slider, but it does not have much effect, and I'm thinking that the darker tone should spread a bit higher up the text.

I checked and it is linear.

To have more control you'd need to make a better gradient preset, or run a gradient up a new layer, and clip it to the Type layer.  Let's try that.

I am still using Classic Gradient and ran it up just the height of the type.  The symbols I have highlighted in green indecate that the gradient is clipped to the layer below it.  You can do this by right clicking the layer and choosing Create Clipping Mask or using the shortcut Alt Ctrl G

You can then move the gradient layer, or useFree Transform to change its spread.

Note, I am ssuming you just want to apply the gradient to the text, and not to all of the yellow graphics as a single unit?

what app?