This effect is caused by chromatic aberration through the lens. An inexpensive microscope will not have a high enough quality lens or the additional (costlier) components necessary to correct this.
Work Step by Step
As discussed on page 728, chromatic aberrations in lenses occur because different colors of light travel at slightly different speeds within glass (this phenomenon is known as dispersion). Thus, the different colors are bent by different amounts. After multicolored light passes through the lens, the different colors will be focused at different positions, and the image appears blurry. Normally, this can be corrected by using different types of glass in a multi-lens system, to partially compensate for the effects, or using low-dispersion glass. However, an inexpensive optics system won’t be carefully designed, and will suffer from chromatic aberrations.