Looking for sketching advice

Hello everyone,

I’m working on improving my sketching this semester and I’m looking for advice on my technique. I did these sketches in 10-15 minutes each. Are these of a reasonable quality for that amount of time? Do you have any other advice? I’m using Pilot Fineliners and Copic markers.

Here are a couple more I drew earlier. I know I messed up on the perspective on the front handle of the top right one, but is there any other constructive criticism you guys can give?

I think the grey scale sketches show a lot more surface differentiation. I’d say to try sticking with grey scale for now and use color to draw attention to key elements of the design such as touch points. Push the light source contrast to make them pop. Work the overall design. Right now the overall designs seem like a collection of disparate elements, work to make them talk to each other as a holistic solution.