Pitcher having trouble seeing fingers of the catcher in poor lighting

Tape the fingers with white tape. If this does not work develop hand signals. Knee, chest, head, etc. The Angels do this quite often.

We do the hand signals as well.

our catchers sometimes paint fingernails. we've also used hand signals. but the nail paint worked well. it was their idea.

Some catchers lean their upper body forward creating even more shadows. Teach them to keep their upper bodies upright to let more light in. May help a bit.

You could do body signs. The catcher can touch his mask, chest, or either shin guards.

There are many ways to make this work. Here are a few examples:

You can subtract the number of touches from 10 and that be the pitch to throw. EX: Catcher touches his body randomly 7 times. 10 - 7 = 3. So the pitcher throws a slider (or whatever his 3 is).

You can also have an indicator. Let's say the indicator is the catcher's chest. You could do the second sign after the indicator or something like that.

And my favorite, but only works for a three or less pitch pitcher is to use the glove while giving fake signs. EX: Catcher touches his mask, chest, and knees randomly but the real sign is where his glove is. If it is on top of his knee it is a fastball, if it is relaxed on the inside of his thigh it's a change up, and if it is on the ground like he is blocking signs between his legs it is a curveball (or whatever the pitcher throws).

Just be creative. It's fun to make up signs and there are plenty of ways to make it easier for the pitcher to see the signs.

Not too mention your defense will know what is coming too so they can play accordingly.

I don't want to sound too silly, but have you had his eyes checked? Just a thought.

Reflective nail polish or reflective tape on the fingers.

I had the same problem and glasses helped me out. But before I got those me and my high school catcher created our own signs. It helped a lot and also had other schools trying to steal our signs and never caught on.