Sky Texture

Yes, you should do it in an image editing program, there will be to much trouble trying to do it in blender.
Basically it is done this way:
- using lasso select tool, rougly select the area you want to color;
- choose two colors;
- use gradient tool;
- change blending mode to the one that gives the best result;
- Filter - Blur - Gaussian blur, blur it a lot;
- if you want to adjust colors, Edit - Adjustments - Hue/saturation, limit the range to the color you want to change