This is an often discussed topic in economic history, and there are two parts to the answer:

1. Colonies have very little observed correlation with GDP growth,
2. The period of positive GDP per capita growth in most of Europe did not start until Spain had already lost its colonies.

Starting with the first part, imperialism often had no economic benefit for the people of the colonizing country. It was certainly sold as a way to close off markets since the creation of mercantilism, and later after the Panic of 1873, but this argument rarely materialized in reality. Colonial ventures often ended in bankruptcy and considerable capital loss for the home country. Examples are the Belgian Congo, initially a semi-private country owned by King Leopold II which was perpetually insolvent (Belgian taxpayers eventually picked up the bill), over a dozen failed French colonial companies in the Sahel, and the famed British East India Company, which after the turn of the 19th century rarely ever turned an annual profit. More egregiously, colonization led to scams that preyed on rich and poor alike, most notoriously the South Sea Company and French Mississippi Company. Other colonial ventures that were marginally profitable, such as the Japanese sugarcane plantations in Taiwan, greatly disappointed the wild expectations of their creditors.There were blowout commercial success stories in colonial ventures. Namely, the famed cash crop islands of the 16th and 17th century, the Spanish gold and silver mines in the Americas (though the import of so much bullion reaped havoc on the Spanish economy), the Austrian Ostend company, and the Japanese Mangyo K.K., currently known as Nissan. However, these successes were the exception and not the rule, and generally their profits were not well spent. Going through the examples I mentioned, nearly all the wealth of the "spice islands" went to a planter elite with negligible benefit to the home economy. Case in point - net GDP per capita growth among all colonial powers in the 16th and 17th century, when cash crop islands were at their peak in economic prominence, averaged zero, and in some cases negative. After Spain lost the Spanish-American war in 1898, the country actually experienced an economic boom as former sugarcane plantation owners returned to Spain and started spending their (much reduced) wealth in the home country. The planter economic model dependend on the constant import of slave labor and accumulation of land - two activities that neither increased consumption in the home country, nor had any effect on the average wage.

The Spanish bullion operation in the 15th and 16th centuries led to massive cash inflows for the Spanish government, but ultimately led to hyperinflation in an economy with a bullion currency and the collapse of currency exchanges in Southern France and Italy. These currency exchanges were tied to the Genoese banks which Spain borrowed from, weakening them. Further, the temporary expansion of crown revenues (from a depletable resource) led to a parallel expansion of expenditures, especially military. This led to ballooning debts and, ironically, to four bankruptcies. Genoese bankers, already weakened by inflation courtesy of their chief borrower, collapsed entirely during the reign of Philip II of Spain, leaving the crown without a line of credit with a manageable interest rate for decades.

The Austrian Ostend company, by all metrics, was a financial success, but illustrated another paradox of imperialism - the requirement to defend its few financial successes. The Ostend company was coveted by Austria's nominal ally, Great Britain, which, in exchange for financial support against Frederick the Great of Prussia and Louis XV of France, forced Empress Maria Theresa to give the company to them. Finally, Mangyo KK's case illustrates another side of this paradox. While profitable (mostly due to the use of forced labor), Nissan's factories were eventually seized and dismantled by the Soviets in 1945 after the failure of the Japanese war effort, leading to an immense financial loss.

The evidence against the financial benefits of imperialism is not just anecdotal. Net GDP per capita growth in France, England, and Spain during the 16th and 17th centuries was zero. During the ascendance of the British Empire after the Congress of Vienna, Britain's GDP per capita growth averaged 1%, a state that continues to present day (uninterrupted by the dissolution of the British Empire).

Why, then, did governments accumulate colonies? Because of political pressures, and because it benefited them. A colony, in absolute terms, gave the state more money, more people, and potentially more soldiers. It made the government more powerful. This potentially surprising conclusion - that colonialism did not benefit the people of the home country but did benefit their governments - is actually quite intuitive. To some extent, all governments depend at least partially on the consent of the people they goverrn, and the history of empires is one of constant compromise with local institutions and leaders throughout the empire to make a multi-national realm governable.

Even if there was a way for empire to benefit the average person in the home country, Spain "missed the boat" on the chance to exploit this. The world as a whole did not experience consistent, positive GDP per capita growth anywhere between 5,000 BC and 1694 AD. In other words, the average Babylonian farmer was financially no worse off than the average English farmer in 1650. Some countries experienced temporary "spikes" and "drops" in their GDP per capita - some examples include ancient Athens, medieval Venice, and Netherlands, all anomalies by virtue of being great commercial centers, which enjoyed periods of comparatively high GDP per capita (as much as $1,500 or more in modern dollars), but these countries too "flatlined" at that level and did not experience significant growth after attaining it.

In 1694, the creation of the Bank of England, a management agency for crown debt, fundamentally changed global financial markets. Before the turn of the 18th century, the rule of financial institutions everywhere was instability. Governments were never accountable to their creditors, and defaults were seen as a part of the business. Banking before 1694 was seen in the same light as playing the stock market today. As a result, the majority of the world's funds went un-deposited (hoarded), and therefore un-invested. Having triumphed over a dynasty associated with absolutist aspirations in 1688, the English parliament, which had been the victim of constant defaults by their monarchy, ensured their new King's finances would not be self-managed. This stabilized the English financial system and made depositing a safe proposition for the first time, bringing the effective interest rate on government bonds down from 11% to 3% within a few decades despite a multiplication of the debt and creating a corresponding decline in private sector interest rates.

These institutions gradually spread to the rest of Europe, but on the whole were not adopted by any countries outside the Netherlands and the United States until the turn of the 19th century. Several countries had created national banks (some predating the Bank of England), but none had instituted stringent financial controls like the British had. As a result, net GDP per capita growth in most of continental Europe averaged 0% until the end of the Napoleonic Wars, even though Britain had seen 1% average growth for the better part of a century. By that time, Spain had lost all its major colonies save for Cuba and the Philippines. Even if they could be profitable, it was too late.

One final caveat, however - Spain is not one of the poorest countries in Europe. Compared to Northwestern Europe, its GDP per capita is low, but it is doing well compared to much of the South and East.

Sources:

Johnson, Steven. King Leopold II's Exploitation of the Congo from 1885 to 1908 and its Consequences.Chalmers, Johnson. MITI and the Japanese Boom.

North and Weingast. Constitutions and Commitment <The seminal document on financial reform and the transition from zero growth to positive growth>

Madsen et al. Four Centuries of British Economic Growth: The Roles of Technology and Population.

Elliot, John. Empires of the Atlantic World: Britain and Spain in America 1492-1830.

Firth, S.G. The New Guinea Company, 1885–1899: A case of unprofitable imperialism.

Fichter, James. British and French Colonialism in Africa, Asia and the Middle East.

Pommeranz, Kenneth. The Great Divergence.

Acemoglu et al. The colonial origins of comparative development: An empirical investigation.

Feyrer and Sacerdote. Colonialism and modern income: Islands as natural experiments.