Ok, I'm back!
There are really two aspects to this, the internal (diet) and the external (hair care etc).
Where diet is concerned, the most important food group for hair is protein, as hair is made up mostly of a type of protein called keratin. You need a good intake of the essential amino acids in order for the body to produce a good quality, strong hair, that will be less liable to break. The best source is first class protein (meat, poultry, fish & eggs), but vegetarian alternative such as beans, pulses, lentils & tofu are good too. You should aim for 2 portions per day. Red meat is an excellent source of readily absorbable vitamin B12 and iron, which are essential nutrients for optimum hair growth, so try to eat this once a week.
You should try not to go longer than 4 hours without food, as the hair follicle will start to become depleted of nutrients after this time. Hair cells are the second fastest dividing cells in the body (after bone marrow), so their demand for energy is extremely high, hence a poor diet will often be reflected in the hair.
With regard to supplements, my short answer would be don't waste your money. When people come to see me in practice, I often recommend blood tests, and where deficiencies show up, specific supplementation may be recommended. Otherwise, the best thing is to aim for a healthy, balanced diet, unless there are obvious exclusions from the diet.
Remember that changes to your diet can only affect the new hair growth, as once the hair has emerged from the scalp, it's dead. Hair only grows half an inch per month, so sadly it will take a while to see the results of these changes!
Now to the external. Unless your hair is in absolutely dreadful condition to the point of snapping off, I would say you are fine to continue to use colour. The psychological side is important too here, if you do not want to be grey! You just have to make sure you take extra care of your hair to counteract the inevitable drying effect of the colouring process. The advice up-thread to use conditioning masks and oils is good, as well as a nice moisturising conditioner after every shampoo. Always just have the regrowth coloured, so you are not colouring on top of colour.
In terms of washing, it is fine to wash your hair every day! It is a total myth that this is a bad thing. As long as you are using a shampoo that is sufficiently gentle, no damage will occur. Never pile the lengths of long hair up on top of your head to shampoo, as this can lead to tangles & matting. Just let the suds run down the lengths. As mentioned above, condition only the mid lengths & ends, as the hair may look lank if you condition right up to the roots. Ensure the conditioner reaches all your hair, sometimes a comb can be helpful to ensure an even spread before rinsing.
Always use a comb rather than a brush when the hair is wet, as the hair is much more vulnerable to damage when wet. You should ensure your comb is saw cut, rather than made in a mould in 2 pieces (check between the teeth, if you can see a join, this is bad!). As mentioned above, start near the ends and gradually comb from higher up.
Now to drying. Blow drying is actually not too bad for the hair, providing the setting is not too hot & not held too close. However, by far the more damaging element is the tools used. NEVER use a bristle brush! These are the worst thing for your hair, they will tear it apart. Use a brush with plastic prongs. Also to note here, a brush will not stimulate the scalp or distribute oils, these are also myths. If you leave your hair to get very greasy, you will find this has no bearing on your dry ends! The oils do not travel down the hair shaft. Scalp massage will also not stimulate growth, as hair loss/thinning is not caused by a lack of blood flow (this was debunked about 100 years ago!).
Lastly, don't use hair straighteners. They are the single worst thing you can use on your hair, and I have seen dreadful damage to people's hair from their use. They are often viewed as not much different to using a hairdryer, but they are much hotter, are in direct contact with the hair, & there is the added friction of dragging them over your hair. The key to achieving long hair is keeping it in good condition, and this will be far less likely if straighteners are used.
Obviously the ideal is to leave the hair to dry naturally, but many people either don't find this practical, or don't like the look of their hair when dried this way. Certainly if you can do it occasionally, it would be good.
With regard to trims, only have enough trimmed to remove any obvious breakage/spilt ends. If you leave bad spilt ends, they may continue to spilt further up the hair.
The only aspect of your op I can't comment on is where you mention your hair is thinner. There can be so many reasons for this, I couldn't comment without seeing someone in person. However, I hope you might find some of the above useful.