Can I use off color yarn to join granny squares for 9 patch.

That's the beauty of granny squares, anything goes. Personally, I prefer black to join it all together but frankly anything will work. Be proud of your work! Congratulations. That's quite an accomplishment Judy.

snything will work but you loose your pattern by using different colors. I'd stick with a black or other solid color

Sure, you can use any color that will coordinate with your bedroom. It will look best with a solid color, but you can even join groups of nine patches with one color and then join them together with another color. One way to decide is to play around with colored pencils and graph paper to come up with a color scheme that YOU like.

I have found the best look is when the last row of all squares is the same color and joined with the same color. If you have already finished all the squares just add another row so the last row is all the same color.

Grandma used to join her pieces together with black yarn & then edge the whole thing in black yarn [a least 2 rows of crochet], she preferred black because it makes the other colors stand out. But, I would use a color that matches the general décor of the room.