Here's a bit of a different perspective.
If all you are doing is recording riffs to give your band, then it doesn't matter what you record it on - and why are you worried about it coloring the sound? For guitar parts, color is what counts (at least to me, because I see music in colors in my mind). However, the color I am talking about is affected by EQ, effects, playing techniques, string condition, pickups, etc... Get the sound you want to get out of your amp and then record it, don't worry about whether or not it is colored. If you like the way it sounds, then it sounds OK for your purposes. And why are you worrying about mixing if all you are recording are riffs? Also, for what you are describing, headphones will do just fine, however, for critical mixing, don't do it. I have tried with
Sennheiser HD600's and it doesn't work well for mixing more than a couple of sounds.
Some of Pink Floyds best songs were first recorded on a small reel-to-reel tape deck (no, not one like you would find in a studio, just one of those old decks, possibly even in mono, that people had to listen to) before they were ever recorded in a studio. I used to hook up my amp to a stereo cassette deck and record riffs like that. If you doubt me then go get a copy of the Making of The Dark Side of the Moon DVD.
You don't need fancy monitors to learn about EQ. Start with what you have, the EQ on your amp, or your stereo. You might find a home hi-fi EQ at a thrift store that will let you experiment. You can get EQ stomp boxes also, or you can download something like Audacity and experiment with EQ on your computer. Just experiment with it and listen to how it affects things.
It is easy to say that you need good monitors, but good monitors are only as good as the gear they are hooked up to. Even with good monitors you will not get the best results if you have cheap cables, or a home stereo powering them. Every upgrade I have ever made in my studio has made a noticable improvement, whether it was monitors, cables, mixers, sound cards, etc... All of these things work together. If all you have is a home stereo, then there is no point in even considering studio monitors. Hooking them up to that would be a waste of money. If you can afford it and really want to step up, then at least get a small 4-track ($100.00), a small EQ like one of those little Sonitus ones and some active monitors (you can find decent small ones for less than 300.00 that might suit your needs). Just remember, that once you start to upgrade you will worry more and more about room acoustics, the actual quality of the gear you have, etc... and unless you have decent RECORDING equipment hooked up to those monitors with decent cables then they won't put out the sound they are capable of anyhow. Getting really good quality in a home studio does not come without an often hefty price tag.
I guess what I am trying to say is that if you can't afford to invest in some decent gear then use what you have in whatever configuration sounds best to you. Don't expect too much from it and just have fun.
