It is quite normal for meth to turn caramel colour as you smoke a bowl. Even if you have the most pure stuff and acetone wash it, you will eventually get discolouration as you smoke it no matter how careful you are. Normally I find my crack back stays white through about 1/2 the bowl and then starts to go caramel and finally to honey coloured at the end - but I am a very fast smoker and don’t often let the puddle cool right down to solid. So long as it does not taste burnt, you have not gone too far with heating.
If you want to smoke well look into using a spirit burner with a very narrow wick and small flame. They can be home made using a perfume bottle and a ball-inflator pump spike, or you can buy them on Amazon (with slightly larger flame). They are fuelled with methylated spirits/ denatured alcohol. On the lowest possible flame setting they only burn at around 150 C which is just below the melting point of meth. You can keep the flame right on the bowl almost the whole time and the meth won’t burn. It makes you smoke slower too. Way better than either a bic or torch.