Name of indicator :
Blue and red litmus paper (they’re natural indicators by the way, you can use methyl orange/phenolphthalein as well but litmus paper would be much easier)
If I were you, I would plan the experiment like that :
Procedure :
1. Prepare two test tubes and some blue and red litmus papers. Label the first test tube as A and the second one as B.
2. Pour the acid into test tube A and alkali into test tube B.
3. Using a dropper, extract some of the acid from test tube A and put a few drops on the blue and red litmus paper.
4. Record your observation.
5. Repeat steps 3 to 4 by using the alkali from test tube B.
The result with acid (Test tube A) :
The BLUE litmus paper turns RED. However the RED litmus paper DOES NOT show any changes.
The result with alkali (Test tube B) :
The BLUE litmus paper DOES NOT show any changes. However, the RED litmus paper changes to BLUE.
(I might made some mistakes here…I’m not confident with my answer but I hope it helps!)