I really like what you done in that short time in hoby. Maybe you and others will accept some tips.
Painting miniatures is not same as real big objects, which have natural lights and shadows even the surface is painted with one color. You must support light with some shading and highlighting on small scale models. You must paint some highlights and shadows yourself directly on the model for better and more realistic looks. I'm in hobby since 2010 and I'm still learning. I can say, if something looks imposible for you now, it will be normal for you in few years. So if some my advice will be too hard to execute, just keep trying.
This is my version of Cadre Fireblade from 2013(so after 3years in hobby) just after releasing of the model. As you can see, I used some trics to make model more interesting.Cadre Fireblade
by Tomáš Pekař
, on Flickr (image is clickable and if you click on the picture after redirect, you will see better picture)
1. Edge highlighting(EH). Line should be thin, precise and consistent as much as you are able to. It is better to use more than one color for EH. I use up to 3 colors.
The color of first layer is little bit lighter than the base color of the armor. I paint all edges with this color so I mean all armor plate have edges painted by this color, no exceptions(sometimes the armor part is too tinny and EH left no base armor color).
The second color layer of EH is the tricky one that improve your model more than you expect. The color of this layer is something between the color of first layer of EH and white. Painting of this layer need some practice to master, but tau models are idelal for it, lots of edges
. You must chose some rules of directing of light for it and need follow these rules all over model. For example if you chose that light will be top left in front of the model from your point of view, all edges faced to this ligh must be highlighted with the color of the second layer. Simply highlight the top and left edge of the all areas. Often you realize that edges do not have ideal direction against your light source. In this cases you need second rule. The part of the edge close to your light source must be lighter than part of the edge far from the light source. So often you paint second layer of EH only on the part of the edge. It is important if you do not finish second highlight on the end of edge to smoot transition between this layer of EH and previous one. No sharp ending of highlights in the middle of the edge! Intensity of second highlights is only on taste of every painter. Other edges that are not faced to the light source are not painted with the second layer of the EH. In my example it is the bottom and right edge of areas.
The color of third layer is white, or color very close to it. This layer is very useful if you want enhance sharpness of the edge. More white, more sharp. It consists from dots or very short lines of color on parts of edges the most close to you light source. Mostly on the joints of edges. In my example you make dot on the join of top and left edge.
2. Area shadding and highlighting(AS). The rules for shadding and highlighting of areas are the same as for EH, it must be, if you want consistency over the whole model. Closest part of the area is lighter than the rest of the area. Areas faced to the lifgt source are lighter than areas aren't faced to light source. I used both balanced, shadding and highlighting, on my Fireblade or characters. On regular firewariors I used only very decent highlights and more shadows. It is faster to paint and character stands out from the squads. GW and 'Eavy Metal team do not use too much AS for tau models, so it can be skipped for save some time but remember, do AS before EH
On this picture you can result after using both these steps, EH and AS(if I remember only shadows are used here). Left drone is after basecoat. Right one is different drone with fully painted disc. Result is much interesting and eye catching model.Drones WIP
by Tomáš Pekař
, on Flickr
I think these two techniques are basement for every painter. Of course you can use lots of different techniques to make model more realistic. Like different style of highlighting in scale modelling, battle damage, rust and dust techniques, water efects but I recomend leave them for later.
I hope it helps and sorry for my English. Good luck.