This is the way the new background should look like:
On a new layer we’ll represent the grass. In this case we’ll select the Brush Tool (B) and two standard brushes of two types, as it is indicated below:
Set both black colors on the tools’ panel:
… and represent the grass the next picture indicates it:
Continue by representing the rainbow, drawing firstly a line by applying the same method we did with the background:
Set the next indicated parameters:
We’ve got a rainbow line:
Create a new layer, marking it on the layers’ palette both with the line’s layer. Press Ctrl+E for the layers to be put together:
Select after that Filter>Distort>Shear to modify the line as it is shown below:
You must get the same thing:
Turn it over with Free Transform option (Ctrl+T)
Increase the lightness (Ctrl+U):
You must get the next result on this stage:
Using the brush of yellow color, try to add on the picture more mist between the layer with the grass and the one with the rainbow:
Applying the Rectangle Tool (U), make a new rectangle, applying the next Blending Options:
It’s the result we must get, but the color of the lines is not so important:
Provide the same rasterizing operation that we did with the rainbow’s layer. Press Ctrl+Shift+U, making it this way a monochromatic one.
Set the same Blending Mode shown below:
Applying the Eraser Tool (E), try to move off those parts of the layer demonstrated next picture:
Make a spot of white color, choosing the brush for this operation:
Select the Paint Bucket Tool (G) to fill the new layer with white color and then insert on the layers’ panel the mask containing the cloud effect (Filter>Render>Clouds)