To answer your question; you can use other colors to complete Drawabox so long as they are dark, easy to see and not overly garish (hot pink, yellow, for example). So long as you are using a fine liner to complete your homework, which you can always double check by asking the community either here or on the discord server.

Starting with your super-imposed lines I can see you do a fairly good job of pushing through and creating your marks with confidence. In your ghosted lines your lines are smooth, confident and committed. There is some arching to many of your lines here though, which you can read about here. Much of the same can be said about your planes homework. your lines are again, smooth and confident looking (although many are still arcing) and your planes are thoughtfully constructed and I can see you did your best on this and the rest of your homework. I do notice that you have some trouble ending your lines where you want them. One thing you can try is when you want to end your mark, lift the pen off of the page instead of trying to stop the motion of your arm.

In your tables of ellipses you did a pretty good job of drawing through them correctly. Your ellipses appear fairly confidently shaped with very little wobbling or signs of hesitation. In your ellipses in planes your ellipses maintain that quality and have a good round, confident shape overall. You also did a good job of keeping your ellipses aligned along their minor axis. The quality of your ellipses improves a lot throughout these exercises the only thing I would recommend is that you try to draw through them only twice to help you focus on maintaining that confidence quality.

In your rough perspective homework you did a decent job of placing your boxes correctly. Your horizontal lines stay fairly parallel to your horizon and your vertical lines stay pretty perpendicular. Your lines are also much straighter than before! There are still some areas where your lines still arc, but I can see a lot of improvement on that here. Keep in mind as you progress through Drawabox and begin to construct more complex forms that it is important to put in the time and focus required to execute each line correctly and to apply the ghosting method to every step of the process, as explained here. This will help you improve and maintain the overall quality of your mark making.

The rotated boxes homework is a challenging and complex exercise. You did pretty well for your first attempt. Your boxes are well constructed with straight lines that show almost no signs of arcing, you drew through all of your boxes correctly and I can see you did your best on this assignment. One thing I notice is that while your boxes are well constructed and you kept the gaps between them narrow and consistent, you did not get enough rotation in your boxes along the major axes. This is a common mistake that you can read about here. Other than that this is a pretty good first attempt that shows a lot of the overall improvement you have made throughout the lesson.

Finally with your organic perspective boxes I can see that you really put in a lot of effort. There is some room for improvement with getting your sets of parallel lines to converge more consistently towards their shared vanishing points. That is something you will work on more in the next steps. Also, remember that while working through Drawabox you do not want to go back over any of your marks in an attempt to "correct" them. The confidence of the stroke is far and away your top priority. Accuracy is something that you will improve on as you continue working through Drawabox and practice ghosting.

In situations where you are attempting to add weight to your lines it is important that you treat the added weight the same way you would a brand new line. That means taking your time to plan and ghost through your mark so that when you go to execute it the mark blends seamlessly with your previous mark. This will allow you to build and create more subtle and clean looking weight to your lines.

Keep in mind what I have said here and be sure to visit all of the links I have left you above. Good luck with the 250 Box Challenge!