The thing with the black and gray walls is more related to verifying printed proofs or for example, calibrating a picture with its version on screen. Yes your yellow walls will affect how you see colors and corrupt your interpretation of them. But you need to keep in mind that if you're not using any proofing system and you plan to approve the proofs at the printer instead, you can always approve the proofs in the printer's "calibrated rooms" to be sure.
Even for them, it's very hard to get to set up the perfect environment for this as many companies have to use industrial fluorescent lightning that make color more bluish; the real 5k light bulbs are way too expensive to be used everywhere. So one way they make sure to have the perfect lightning to verify colors is by creating a box for this where they can add the perfect 5k light bulb too. It's usually a small black box with the 5k light, and they look at the printed proof in it. The bigger place will have an entire small room set up like this. If they do color management of high level, they will set up these computers in that environment but rarely the whole design team.
Ideally, you should have walls and curtains with neutral tones; they can be light gray or white. If you notice that your walls reflect a lot of yellow, that means you might want to modify the paint on your walls.
About the red shirts, maybe they mean you shouldn't wear bright colored clothes when you calibrate or do projects that require very precise color adjustment (eg. light colors.) Otherwise that's a bit overkill for freelance work. It's true though that you should avoid buying anything too colorful for your office for now on (eg. bright colored furniture or accessories.) The tips you've heard were good but targeting hardcore color management and calibration, and before you start changing shirts to work, you might want to simply get the right tools to calibrate your display properly, paint those walls and fix the light in your room. If you can't buy the X-Rite then at least fix your walls and lightning to be more neutral. The goal is to work in a neutral environment.
Regarding the light, you can buy these 5k light bulbs but just so you know, they not very far from the standard light bulbs; just don't buy the "energy saving" type, they're like fluorescent. The "ideal" light for color is the noon light at 12 and that's the condition you need to try to reproduce in your own office. That's what the 5k light bulb actually help for. For your web projects, it's not as important as long you're in similar conditions as the people who will look at your apps and websites.
When you might want to be careful is when you'll work on very light skin or sand tones, or any light color that have very low percentages of CMYK. You'll end up learning how to trust the CMYK numbers instead of trusting only what you see on the screen. You'll also end up learning that nothing is ever perfect and the goal is always to get as close as possible to perfection; you won't be able to control the 1-2% "too-much-cyan" on the press but you'll know how to choose your colors properly to compensate for all the little imperfections of printing! Or simply choose to print with a Pantone.
If your work requires you to do a lot of skin tones or broken white projects, for example wedding catalogs and magazines, then yes you should really try to get to the optimal conditions.
Now this is a personal suggestion; you don't need to go crazy with all this but yes you should try to get closer to an ideal. And for your proofs, approve them outside under the sun if you're not certain or ask your printer to go in one of their room that is adjusted for verifying proofs.