writing down the indicators they test:
colourless [12.0-14.0] orange
colourless [8.3-10.0] pink
yellow [6.4-8.0] red
blue-violet [3.0-5.0.] red
assuming that a 'red' solution means purely red, the first two must be colourless, i.e. pH<8.3, and the other two need to be ensured as both red, i.e...