Yes a basking bulb will give you different temps than a regular round style bulb. Not because of the color but because of the shape.

A basking bulb has a focused heat beam causing less heat to spill out and therefore creates a warmer hotspot.

A regular style bulb spills the heat out in a wide beam and therefore does not create a hotspot that is as warm.

I would use the round style bulb since you already have the uth hotspot.

I like temp guns but you can buy them much cheaper on Amazon.

Riv,
The uth does not change your ambient temp. Also a 75 watt ceramic bulb is recommended for a forty gallon tank. I use one in my forty gallon tank in a room that is around 68. I have mine on a lamp stand which gives it a standoff of aprox 24 inches and it creates a hotspot of 89 on the substrate and ambient temps around 80. I Would check your box and see what your 100 watt is recommended for. You can probably buy the cheaper 75 watt in the future.

Surface temp and ambient temp is two different things. Surface temp is just that, the temp on the surface. Ambient temps are the temps of the air in the cage. If you ever get an ambient temp near 90 your surface temps will probably be way to hot.