The color of Co-doped ZnO should be different from ZnO, because Co create additional energy states which provide optical transitions. Also particle size and film thickness affect the color too.

Thanks. I want to know what can I do to decease particle size for disappearing red color. I think because of this color, Zinc ions don't replace with cobalt ions and I have composite instead of doped ZnO.

if you ant to prepare Co-ZnO have to be care about the amount of CO to Zn . ZnO is base which it can be a higher % and Co will be as impurity. however  can be select the CO as wt % from 0.00 wt. % to 2 wt. % and then you can study clearly all properties of CO-ZnO using XRD ,UV......... At same time for your case the  Zinc ions don't replace with cobalt ions becuse may be the amount very high which it make saturated solution between co and zn . try do this as i suggestion to you . good luck

Check this paper, i used SnO2 you can replace tin by znic

Article Modification of the structural and optical properties of tin...