About a year ago i used to have a blonde fringe and black hair, then i got rid of it and went to all black. i had it all one color for ages.
about 4 months ago i went back to a blonde fringe and it was fine for ages, then i tryed to dye it red and the roots went red but the ends stayed blonde.
so i thought sod it so iv dyed it all one color again Blue/black but my fringe is tyrning gray now.
what can i do to sort it??
im gonna try a normal black hair dye on it one last time.
dose it just need cutting now or can't i save my hair??

p.s sorry for going on a bit lol

You can't dye already-dyed hair by simply using another, lighter, hair dye. You have to remove the artificial color, first. Some people call it "stripping", though that usually refers to the use of color removers containing bleach.

More modern color removers dissolve the artificial pigment and allow it to be washed out. These color removers are sulfur-based (they, therefore, have a smell similar to permanent wave solutions). Some common brand names are ColorFix, Color Zap, Color Oops, Color Charm Color Corrector, and Igora Phantom.

Once you've removed the artificial color, the hair will not look very nice. What you'll see is the hair with the remaining natural "undertones". The "developer" in the haircolor mixture lightens and discolors some of the natural pigments even in black colors. So, you obviously have to re-dye it using a more reasonable color.

All of that is for going lighter. If you're going back to black, you shouldn't have too many problems as black will pretty much dye over anything.

Now, one more caveat: Single-process haircoloring is limited in its ability to lighten. That's why you never get the color of the swatch or on the box. That's where hair bleach comes in. You can use a double-process (bleaching and toning) to get lighter colors. But, that's another story.