Cardboard pH Indicator Formulation

I’m developing an ultra-low-cost cardboard strip that changes colour between pH 6.0 and 8.0. I need a practical indicator solution that I can mix myself, then apply to plain card in the simplest way possible.

Cost-effectiveness is the key driver, so feel free to choose whichever dye or blend—Methyl Red, Bromothymol Blue, Nitrazine Yellow, or any comparable alternative—gives a clear, repeatable transition across that narrow range without expensive reagents.

What I need from you:
• Exact formulation: solvent, indicator concentrations, optional stabilisers or binders that won’t weaken the cardboard.
• Application guidance: outline the single easiest method (spray, dip, brush, or another idea) with drying time and safety notes.
• Compatibility check: confirm whether a light dusting of sodium bicarbonate on the card will affect readings and suggest fixes if it does.
• Simple reference chart: expected colours at 6.0, 6.5, 7.0, 7.5 and 8.0 for quick visual matching.

Keep ingredients readily available, instructions concise, and the overall setup economical. Once I have those details I can move straight into small-scale production tests.

Stability is important, and the pH color should remain the same for at least 2 weeks.