Connecting Arduino or ATtiny85 to a Smartphone via Bluetooth

Is it possible to pull information from an iPhone or Android App to change the color of an LED on a Android (Uno) or ATtiny85 run device?

I am looking to make a "Toastmasters Timer" for speech contests. There is an Smartphone App that changes color for different timed speeches to let the presenter know when they reach their time limit.

I wanted to make a small device with a ATtiny85 (to reduce size) inside & some RGB LEDs that I can place on the table. I was thinking of using Bluetooth to communicate between the two devices.

Thank You!!

Why the need for an app?

Can’t you do a stopwatch type of code?

Do HC05=>Attiny86 using the softwareSerial on 2 pins ,you have 3 pins left for your LED thing