You can keep a face, an outfit and a setting the same and still end up with a character who looks like a stranger, simply because the hair changed. Hair is one of the details that drifts most easily between pictures. A clear, steady description keeps it under control.
Start with length
Say where the hair ends. Chin length, shoulder length and waist length are easy to picture and hard to misread. Words like long or short leave too much room, and the result can change a great deal from one picture to the next.
Name one clear colour
Choose a simple colour and stick with it. Dark brown, platinum blonde or copper red all work well. Avoid mixing several shades in one description unless highlights really matter to the look, because extra colours tend to wander.
Describe the style simply
Straight, wavy or curly, worn loose, in a ponytail or pinned up. One or two words for texture and one for how it is worn is usually enough. Long chains of style words often confuse rather than help.
Use the same words every time
Once you have a description you like, copy it exactly into each new picture. Small changes in wording can lead to bigger changes in the result. Keeping a short note of your character details makes this easy.
Change it on purpose
If you want a new look, change one detail at a time, such as putting the hair up for an evening scene. Keep length and colour the same so your character is still clearly themselves, just styled differently for the moment.