Best way to color very short hair -advice pls

My hair is very short (as in 1 about 1 1/4" in top and 1/2" on the sides). I color my hair at home and before it was so short it was easy to apply new color to the roots then color the ramaining the last 10 minutes. Now its so short it is almost impossible to just get the roots as the color seems to travel all the way to the ends anyway.

For super short hair is it OK to just do a whole head treatment, I almost do it now anyway without trying! Please advise me what the proper way to do it is! Thanks! (ps. I color every 4 to 5 weeks)

Thanks!